Refs #342: Document incoming and outgoing messages

- Add doc for incoming message and outgoing messages
- Add doc for other classes referenced by them
- remove meaningless `sent_at` and `received_at` attributes from messages
Merge remote-tracking branch 'remotes/origin/develop' into 342-document-incoming-outgoing-messages

Conflicts:
	rapidsms/messages/base.py
	rapidsms/messages/incoming.py
	rapidsms/messages/outgoing.py
Merge pull request #342 from rapidsms/342-document-incoming-outgoing-messages

Document incoming and outgoing messages