Verification of message signatures against segwit addresses
is not currently functional/possible in Core, and additionally
the signing function used for messages in jmbitcoin, derived
from coincurve, was not compatible with Electrum either.
This commit uses the functionality in python-bitcointx's
signmessage module, and now the wallet method `signmessage`
creates signatures on messages against segwit addresses that
are verifiable in Electrum.
Test cases of p2sh and native are added.
Adam Gibson
Kristaps Kaupe