Signing <hexdata> with <wif> private key.

def sign_data(self, wif, hexdata):
        """Signing <hexdata> with <wif> private key."""
        data = deserialize.binary(hexdata)
        key = deserialize.key(self.testnet, wif)
        sigdata = control.sign_data(self.testnet, data, key)
        return serialize.signature(sigdata)