doc.go file started

Fixes #35
Updates https://github.com/tendermint/coding/issues/27

Started a doc.go file to provide an overview/high level
dive into the functionality of this repo.

Also added an example_test.go file in which we can
put end-to-end code examples/actual usage patterns
that can be copied and pasted and will always have to
compile when tests are run to ensure that we don't regress.

doc.go

@@ -0,0 +1,48 @@
+/*
+go-crypto is a customized/convenience cryptography package
+for supporting Tendermint.
+
+It wraps select functionality of equivalent functions in the
+Go standard library, for easy usage with our libraries.
+
+Keys:
+
+All key generation functions return an instance of the PrivKey interface
+which implements methods
+
+    AssertIsPrivKeyInner()
+    Bytes() []byte
+    Sign(msg []byte) Signature
+    PubKey() PubKey
+    Equals(PrivKey) bool
+    Wrap() PrivKey
+
+From the above method we can:
+a) Retrieve the public key if needed
+
+    pubKey := key.PubKey()
+
+For example:
+    privKey, err := crypto.GenPrivKeyEd25519()
+    if err != nil {
+	...
+    }
+    pubKey := privKey.PubKey()
+    ...
+    // And then you can use the private and public key
+    doSomething(privKey, pubKey)
+
+
+We also provide hashing wrappers around algorithms:
+
+Sha256
+    sum := crypto.Sha256([]byte("This is Tendermint"))
+    fmt.Printf("%x\n", sum)
+
+Ripemd160
+    sum := crypto.Ripemd160([]byte("This is consensus"))
+    fmt.Printf("%x\n", sum)
+*/
+package crypto
+
+// TODO: Add more docs in here

example_test.go

@@ -0,0 +1,35 @@
+// Copyright 2017 Tendermint. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package crypto_test
+
+import (
+	"fmt"
+
+	"github.com/tendermint/go-crypto"
+)
+
+func Example_Sha256() {
+	sum := crypto.Sha256([]byte("This is Tendermint"))
+	fmt.Printf("%x\n", sum)
+	// Output:
+	// f91afb642f3d1c87c17eb01aae5cb65c242dfdbe7cf1066cc260f4ce5d33b94e
+}
+
+func Example_Ripemd160() {
+	sum := crypto.Ripemd160([]byte("This is Tendermint"))
+	fmt.Printf("%x\n", sum)
+	// Output:
+	// 051e22663e8f0fd2f2302f1210f954adff009005
+}