Generalizable serialization format

[dcousens]

Related to #428, would make patching in alternate crypto currencies much simpler.
Could potentially allow for stricter and more deterministic type checking.

I don't want to over-generalize though, and if this was to be done, it'd definitely be implemented/tested through an external module, not in bitcoinjs-lib.

edit: Should also support encoding of valueBuffer natively

• Would also rename internal variables more consistently (aka .vout instead of .index for Transaction Inputs #622)

[jprichardson]

I'm definitely in favor of this and more than happy to take the lead on this. This is something that I definitely need.

[dcousens]

@jprichardson taking your time on this one? 😆
Going to un-assign for now