* RPC method parity_clearEngineSigner
Add RPC method parity_clearEngineSigner
Fixes https://github.com/poanetwork/parity-ethereum/issues/113
* corrected the return type of clear_author
* review comment responses and a rebase fix
* removed a spurrious warning
* moved clear_signer functionality to set_signer
* Merge clear_author into MinerService::set_author.
* Add trace logs to Clique::set_signer.
* Clique: Don't lock signer multiple times.