Added verifyMessage support (already in plugin/firmware)

signature and message have to be in Hex format, not sure if add automatic conversion or not.
It returns a "Message verified" on success or "Signature invalid" on fail.
Addded signMessage thanks to prusnak : https://github.com/conejoninja/trezor.js/commit/f42c1de44aea1066a75e348789acf9c171881ccc#commitcomment-7690147
Merge pull request #2 from conejoninja/verifyMessage

Added signMessage and verifyMessage support