Misc. updates in response to bulk messaging pull request comments
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
Refs #350: Changes as a result of testing rapidsms-appointments

    - msg.responses now contains dictionaries, not messages
    - cope with apps using old `connection` parameter for messages for now,
      but issue deprecation warning
    - allow for djappsettings not being installed
    - issue errors for some old ways of calling things that we can't handle
      anymore.
Merge pull request #357 from rapidsms/bulk-api-comments

Misc. updates in response to bulk messaging pull request comments
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
Merge remote-tracking branch 'remotes/origin/develop' into 350-test-other-apps
Merge remote-tracking branch 'remotes/origin/develop' into 350-test-other-apps

Conflicts:
	rapidsms/messages/base.py
	rapidsms/messages/incoming.py
Merge pull request #350 from rapidsms/350-test-other-apps

Try 0.13 with an existing app
Merge branch 'develop' into 359-release-checklist

Conflicts:
	docs/index.rst