Bug fixing and cleanups

[adigeo]

Allow SIP core engine to be restarted by properly deallocating memory.

Done code clean-ups/enhancements:

1. We need a solution to accept a session without a 180- ringing. For instance we can have automatic answer based on various preferences.
2. It seems that the stun servers are learned from the DNS and cannot be overwritten in settings, they show up automatically
3. remove Engine.start_cfg
4. get rid of obsolete modules and classes
5. Enable logging of notifications to file
6. Check the validity of the TLS settings/files before starting the engine
7. Crash when more than 32 tones in the tone generator queue (tone-gen removed)
8. remove sipsimple.util.NotificationHandler
9. remove keyword_handler from sipsimple.primitives
10. Move XML payload definitions to somewhere else