implement verifymessage

Moving this intp btcd rather than btcwallet. I'll send a PR to remove from btcwallet once this has been merge. See discussion at: https://github.com/conformal/btcwallet/pull/124#issuecomment-52983230

Question: what is your procedure for testing? I noticed that there is no rpcserver_test.go file.
Done. Probably should add the same check in btcwallet, for consistency: https://github.com/conformal/btcwallet/blob/master/rpcserver.go#L2393