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.
Is a serialized pubkey supposed to be a valid parameter? In Bitcoin Core, it does not seem to be. This is not part of the core consensus code, however, so it may be fine to deviate; let me know what you think.