#173: Make Transaction multi-sign by default, and add Tx.check_signatures() test on multi-sig tx

• Make Transaction multi-sign by default

Store SigningKey and Signature arguments/attributes in Lists. Overwrite Document.{sign(),check_signature()} as undefined.

Disable pylint error at Transaction class scope: https://stackoverflow.com/a/22224042 Because of NotImplementedError exception, not happening with Exception

• Add multi-sig tx test from g1-test tx

Close #173 (closed).