refactoring: transaction class hierarchy

[eckelj]

Description

The goal of this epic is to move the transactions module to it's own package so it can be imported for both planetmint and planetmint-python-driver. For this the logic of transactions and planetmint itself need to be decoupled.

The Transaction and its derived classes are responsible for to instantiating, validating and signing transactions objects. However, the classes contain methods that should be either in the backend, business logic of Planetmint or are solely used for test cases. These should be moved their respective places.

The current approach would be to evaltuate the methods and get closer to a MVC model.

ToDos

• move Create.generate() to test/utils.py
• move Transfer.generate() to test/utils.py
• remove Election.create() and Election.transfer() (unused code)
• move Election methods that are db related to backend queries to planetmint/backend
• move other Election methods to planetmint
• implement new structure in codebase (WIP)