Transaction Builder

The aforementioned, and still in need of plenty of test fixtures, `TransactionBuilder`.

Anyone is welcome to throw fixtures at it, but, presuming you abide by the API (should be obvious), it should automagically sign them correctly with the private keys provided.
See tests for the intended behaviour.

Currently supported is:

* pubKeyHash
* multisig

And P2SH variants thereof. 
Adding support for the others should be incredibly simple due to the design however.

It also needs a `fromTransaction` method for initializing it from an incomplete `Transaction` so you can pass a incomplete `Transaction` around between signers.