[mod] #173: Make Transaction handy with single and multi-sign

Store SigningKey and Signature arguments/attributes in Lists.
Allow to pass an instance or a list of SigningKey and pubkey
for single and multi signing
in Transaction.{__init__(),multi_sign(),check_signatures()}
if instance: store in a list

Overwrite Document.{sign(),check_signature()} as undefined.
Disable pylint error at Transaction class scope:
Raise Exception instead of NotImplentedError to not having to disable pylint:W0223
https://stackoverflow.com/a/22224042

Do not erase previous signatures when signing
To allow different key owner to sign additonally the Tx doc