Relax IsStandard rules for pay-to-script-hash transactions

Relax the AreInputsStandard() tests for P2SH transactions --
allow any Script in a P2SH transaction to be relayed/mined,
as long as it has 15 or fewer signature operations.

Rationale: https://gist.github.com/gavinandresen/88be40c141bc67acb247

I don't have an easy way to test this, but the code changes are
straightforward and I've updated the AreInputsStandard unit tests.