Add support for verifymessage

/api/messages/verify accepts three parameters either via JSON POST data
or GET query parameters, pipes them to the bitcoin RPCs verifymessage
command, and returns the result for the user.
Friendlier error reporting for sendRawTransaction
Merge pull request #233 from koirikivi/verifymessage-broadcasttx

Add support for verifymessage and better error reporting for sendrawtransaction