Block.js

Resolves https://github.com/bitcoinjs/bitcoinjs-lib/issues/279 .

The API right now is very simple, but provides the basic building blocks.
I'm not sure whether we should put merkle tree hashing in here or some where else.

Mostly this abstraction was created to be able to parse blocks, further functionality can be added when necessary?

@weilu thoughts?
My point is that this is a utility function that I expect to write myself as an app dev. I don't think it's relevant to block parsing