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.
Complete nitpick here, but please move the first param up to this line and put the second one on the next line if needed.  This is more consistent with the rest of the codebase.