v1 of making transaction signing code modular

I've abstracted away the scripts into something that any developer can use without having to read/understand a ton of code. This will make it easier for others to contribute to this codebase, as there's now a central place for tx signing. It's still too complicated, but at least it isn't passing around crazy file structures.

I think it'd make more sense to move `bitcoind_signrawtransaction` to `pycoin.services.bitcoind` but I just wanted to send this over first.