Preliminary test for P2shsign

[dcousens]

Fixes #143.
Adheres to #139, and another small nudge towards #106.

This pull request removes Transaction.prototype.applyMultisigs as it was a misleading function with a very singular purpose; and adds tests for Transaction.prototype.p2shsign using a pay-to-scriptHash multisig transaction.

Transaction.prototype.setScriptSig was added to replace applyMultisigs in a more general fashion, with the createP2SHMultisigScriptSig step moved outside of the TX implementation into user land.
This adds a smaller burden on the user space code, but provides better flexibility and API coherency.
This is especially true given that the user already has to generate the redeemScript and signatures in a very specific order; why hide the final step behind some unnecessary function.

It is probably not ideal by any means; but I feel it is a step in a better direction.

Some minor fixes in relation to #139 and a stricter API in general meant removing the automatic lifting of types in p2shsign and validateSig.

Thanks to @juliustayl for helping with this.

[weilu]

Just noticed this. p2shsign => p2shSign shall we?

[dcousens]

Rebased, added some basic types checking in signScriptSig, and renamed createInputScript to createPubKeyHashScriptSig.

[kyledrake]

+1

[kyledrake]

@weilu look ok?