all: Apache2.0 LICENSE conformation to source code file headers

[odeke-em]

So I am currently working on adding a doc.go file to go-crypto, and I just noticed that we use the Apache2.0 License in there.

According to the terms of the Apache2.0 license, every source code file SHOULD contain the short header, so that in case code gets intentionally "misused" it is still well protected. Please see http://www.apache.org/dev/apply-license.html#new.

This is what a sample file will look like if we decide to add the short license in all our source files

// 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
}