ECKey: Add method to sign a hash and return a byte array
ECKey: Add method to sign a hash and return a byte array

The new signHash method does the same thing as signMessage, except that
it takes a hash instead of a message string and it returns a byte array
instead of its Base64 representation.

HashEngineering/dashj,HashEngineering/darkcoinj,HashEngineering/dashj,HashEngineering/darkcoinj,HashEngineering/darkcoinj,HashEngineering/dashj,HashEngineering/dashj