WIP: add support for signing/verifying messages

This adds a new Message class with static methods for signing and verifying a
message the same way as bitcoind. (In a nutshell, messages are prepended with
"Bitcoin Signed Message:" before being hashed and signed).

There is one important piece missing ... verifying a signature with an address,
and not a public key. I have not yet implemented this because the cryptography
interface of bitcore does not allow me to derive the public key from a
signature. This will need to be added before verifying from an address is
possible.