Currently IsDirEmpty returns true, err if it encounters
any error after trying to os.Open the directory.
I noticed this while studying the code and recalled a bug
from an earlier project in which doing the exact same thing
on code without permissions would trip out and falsely report
that the directory was empty.
Given demo.go in https://play.golang.org/p/vhTPU2RiCJ

* Demo:
```shell
$ mkdir -p sample-demo/1 && touch sample-demo/2
$ echo "1st round" && go run demo.go sample-demo
$ sudo chown root sample-demo && sudo chmod 0700 sample-demo
$ echo "2nd round" && go run demo.go sample-demo
```

That then prints out
```shell
1st round
original:: empty: false err: <nil>
updated::  empty: false err: <nil>
2nd round
original:: empty: true err: open data/: permission denied
updated::  empty: false err: open data/: permission denied
```

where in "2nd round", the original code falsely reports that
the directory is empty but that's a permission error.

I could write a code test for it, but that test requires me to change
users and switch to root as a Go user so no point in complicating our
tests, but otherwise it is a 1-to-1 translation between shell and Go.

Found common http utils that were being multiply duplicated across
many libraries and since am moving things in basecoin/unstable to
add for more functionality, it's better to put them in one
place.

Utilities and tests added:
- [X] FparseJSON
- [X] FparseAndValidateJSON
- [X] ParseRequestJSON
- [X] ParseAndValidateRequestJSON
- [X] WriteCode
- [X] WriteError
- [X] WriteSuccess
- [X] ErrorResponse

During review from @ethanfrey, made updates:
* Removed tt.want since it was a distraction/artifact that made
the reviewer think the tests weren't testing for both failed
and passed results.
* Added ErrorWithCode as WithCode is a common options pattern
in Go that could cause confusion:
  ErrorWithCode(error, int) ErrorResponse
* Using json.NewDecoder(io.Reader) error instead of
ioutil.ReadAll(io.Reader) to slurp all the bytes.
* Added more test scenarios to achieve 100% coverage of http.go

We use WriteFileAtomic in two places:

```
p2p/addrbook.go
338:    err = cmn.WriteFileAtomic(filePath, jsonBytes, 0644)

types/priv_validator.go
162:    err = WriteFileAtomic(privVal.filePath, jsonBytes, 0600)
```

and we don't need .bak in any of the above. We save priv_validator every
10ms and addrbook every 2 min.

The problem with {filePath}.new is that it is not safe for concurrent
use! Calling this function with the same params results in the following
error:

```
panic: Panicked on a Crisis: rename /root/.tendermint_test/consensus_replay_test/priv_validator.json.new /root/.tendermint_test/consensus_replay_test/priv_validator.json: no such file or directory

goroutine 47860 [running]:
github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.PanicCrisis(0xcba800, 0xc42152d640)
	/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common/errors.go:33 +0x10f
github.com/tendermint/tendermint/types.(*PrivValidator).save(0xc42235f2c0)
	/go/src/github.com/tendermint/tendermint/types/priv_validator.go:165 +0x159
github.com/tendermint/tendermint/types.(*PrivValidator).signBytesHRS(0xc42235f2c0, 0x6, 0x0, 0xc424e88f03, 0xc429908580, 0xca, 0x155, 0x80, 0xc424e88f00, 0x7f4ecafc88d0, ...)
	/go/src/github.com/tendermint/tendermint/types/priv_validator.go:249 +0x2bb
github.com/tendermint/tendermint/types.(*PrivValidator).SignVote(0xc42235f2c0, 0xc4228c7460, 0xf, 0xc424e88f00, 0x0, 0x0)
	/go/src/github.com/tendermint/tendermint/types/priv_validator.go:186 +0x1a2
github.com/tendermint/tendermint/consensus.(*ConsensusState).signVote(0xc424efd520, 0xc400000002, 0xc422d5e3c0, 0x14, 0x20, 0x1, 0xc4247b6560, 0x14, 0x20, 0x0, ...)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1556 +0x35e
github.com/tendermint/tendermint/consensus.(*ConsensusState).signAddVote(0xc424efd520, 0x2, 0xc422d5e3c0, 0x14, 0x20, 0x1, 0xc4247b6560, 0x14, 0x20, 0xc42001b300)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1568 +0x200
github.com/tendermint/tendermint/consensus.(*ConsensusState).enterPrecommit(0xc424efd520, 0x6, 0x0)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1082 +0x13a4
github.com/tendermint/tendermint/consensus.(*ConsensusState).addVote(0xc424efd520, 0xc424e88780, 0x0, 0x0, 0x39, 0x1dc, 0x28c)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1477 +0x1be5
github.com/tendermint/tendermint/consensus.(*ConsensusState).tryAddVote(0xc424efd520, 0xc424e88780, 0x0, 0x0, 0xd7fb00, 0xc42152ce00)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1382 +0x93
github.com/tendermint/tendermint/consensus.(*ConsensusState).handleMsg(0xc424efd520, 0xcb58e0, 0xc42547feb8, 0x0, 0x0, 0x6, 0x0, 0x4, 0xed10ca07e, 0x3077bfea, ...)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:660 +0x9fc
github.com/tendermint/tendermint/consensus.(*ConsensusState).receiveRoutine(0xc424efd520, 0x0)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:615 +0x5f5
created by github.com/tendermint/tendermint/consensus.(*ConsensusState).OnStart
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:332 +0x4a7
exit status 2
FAIL	github.com/tendermint/tendermint/consensus	76.644s
make: *** [test_integrations] Error 1
```

See https://github.com/tendermint/tendermint/pull/568