Implement full (Proof-of-funds)

[raphjaph]

@diehuxx What do you think of this interface for proof-of-funds?

pub fn full_verify(address: &Address, message: &str, to_sign_base64: &str, utxos: BTreeMap<OutPoint, TxOut>)

The same function as full only with an optional parameter specifying the utxos that are being signed for. If utxos contains more than the inputs signed for then it should error. This would prevent the user from thinking that the function is verifying the utxos being passed in.