Authorize state transitions with signed messages and ecrecover

[adklempner]

Huge barrier to user on-boarding of decentralized applications is paying for gas.

Let's add another version of the performStateChange method that accepts two additional parameters: bytes32 hash, bytes sig and uses Zeppelin's ECRecovery library to allow anyone to submit a TX on the signer's behalf.

If this pattern can work in StateMachineBuilder it'll be worth releasing a generic version, probably as a new addition to the OpenZeppelin library.