Fixes #1006

```
goroutine 230 [runnable]:
github.com/cosmos/gaia/vendor/golang.org/x/crypto/ripemd160.(*digest).Sum(0xc420ac2af0, 0x0, 0x0, 0x0, 0xc420c01360, 0xc420c01378, 0x20)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/golang.org/x/crypto/ripemd160/ripemd160.go:119 +0x2c9
github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle.KVPair.Hash(0x49447e8, 0x7, 0x47ff760, 0xc420048d50, 0xc420aba9a0, 0x14, 0x20)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle/simple_tree.go:122 +0x200
github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle.(*KVPair).Hash(0xc4200f7ae0, 0xc420aba9a0, 0x14, 0x20)
	<autogenerated>:1 +0x57
github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle.SimpleHashFromHashables(0xc420111900, 0x9, 0x10, 0x10, 0xc420ad06c0, 0xc420087500)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle/simple_tree.go:87 +0xaa
github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle.SimpleHashFromMap(0xc420b987e0, 0xc420b987e0, 0x4941892, 0x3)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle/simple_tree.go:96 +0x4d
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/types.(*Header).Hash(0xc420a541a0, 0x4034f, 0xc420a28530, 0xa)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/types/block.go:177 +0x54a
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite.Commit.ValidateBasic(0xc420a541a0, 0xc420a1e300, 0xc420a28530, 0xa, 0xb, 0x27)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/commit.go:83 +0x1c9
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/files.(*provider).StoreCommit(0xc4202f2780, 0xc420a541a0, 0xc420a1e300, 0xc420a24870, 0x0, 0x0)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/files/provider.go:71 +0x5e
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite.cacheProvider.StoreCommit(0xc4202f27a0, 0x2, 0x2, 0xc420a541a0, 0xc420a1e300, 0xc420a24870, 0xc420a541a0, 0xc420a1e300)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/provider.go:39 +0x8f
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite.(*cacheProvider).StoreCommit(0xc4202f27c0, 0xc420a541a0, 0xc420a1e300, 0xc420a24870, 0x2, 0xc420a541a0)
	<autogenerated>:1 +0x70
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite.NewInquiring(0xc420990610, 0xa, 0xc420a541a0, 0xc420a1e300, 0xc420a24870, 0x4f3eb80, 0xc4202f27c0, 0x4f3e5c0, 0xc4202f2840, 0xa)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/inquirer.go:29 +0x5c
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client.GetCertifier(0xc420990610, 0xa, 0x4f3eb80, 0xc4202f27c0, 0x4f3e5c0, 0xc4202f2840, 0xc4200f68a0, 0xc420b21888, 0x470eece)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/common.go:47 +0x141
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands.GetCertifier(0x4f46aa0, 0xc4200f68a0, 0xc420b218e0)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/common.go:82 +0x83
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query.GetWithProof(0xc420a260c0, 0x22, 0x30, 0x0, 0x4947dd6, 0xc42016a000, 0xc420b219a0, 0x4389d85, 0x0, 0x0, ...)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query/get.go:73 +0x52
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query.Get(0xc420a260c0, 0x22, 0x30, 0x0, 0x1, 0x0, 0xc420b21a08, 0x43c9c91, 0xc4200a20f0, 0x4947dd6, ...)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query/get.go:64 +0x178
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query.GetParsed(0xc420a260c0, 0x22, 0x30, 0x47d1440, 0xc420b984e0, 0x0, 0x1, 0x30, 0x1d, 0x40)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query/get.go:31 +0x5f
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/modules/coin/rest.doQueryAccount(0x4f3d300, 0xc4201ee0e0, 0xc420111500)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/modules/coin/rest/handlers.go:66 +0x3a5
net/http.HandlerFunc.ServeHTTP(0x4a9d808, 0x4f3d300, 0xc4201ee0e0, 0xc420111500)
	/usr/local/go/src/net/http/server.go:1918 +0x44
github.com/cosmos/gaia/vendor/github.com/gorilla/mux.(*Router).ServeHTTP(0xc4201aab40, 0x4f3d300, 0xc4201ee0e0, 0xc420111500)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/gorilla/mux/mux.go:150 +0xed
net/http.serverHandler.ServeHTTP(0xc4209992b0, 0x4f3d300, 0xc4201ee0e0, 0xc420111300)
	/usr/local/go/src/net/http/server.go:2619 +0xb4
net/http.(*conn).serve(0xc4200b5540, 0x4f3df80, 0xc420086900)
	/usr/local/go/src/net/http/server.go:1801 +0x71d
created by net/http.(*Server).Serve
	/usr/local/go/src/net/http/server.go:2720 +0x288
```

uint64 is considered dangerous. the details will follow in a blog post.

The certifier package is renamed to light. This is more descriptive
especially in the wider blockchain context. Moreover we are building
light-clients using the light package.

This also adds godocs to all exported functions.

Furthermore it introduces some extra error handling. I've added one TODO
where I would like someone else's opinion on how to handle the error.