Mock classes/threads for backend

[PeterSurda]

Create the following mock threads that can be used for UI tests

• [ ] worker (pretend to do PoW and mark message as sent after a while, or pretend the key doesn't exist)
• [ ] Object Processor (pretend objects are arriving from network and insert records into inbox or pubkey tables)
• [ ] Network thread (just return fake connections, bandwidth)

These will then be used for tests so that we have a more lightweight UI testing capability (Qt, kivy, ...) Make them compatible with Python 2 and Python 3