Add sign hash

[richmcateer]

This is useful when you want to sign a message of a soliditySha3 hash with multiple inputs (which can then be validated in a smart contract with ecrecover using the same inputs). In this case, you don't have the original "data" to pass into web3.eth.accounts.sign, so a separate method web3.eth.accounts.signHash makes things easier.

[coveralls]

Coverage increased (+0.02%) to 85.581% when pulling 54d8adf on richmcateer:sign-hash into 2db8312 on ethereum:1.0.