Refs #328: Use DB backend in httptester

- change httptester to use DB backend
- remove httptester's old backend
- update httptester doc
- add notes to 0.13.0 release notes
Refs #328: Cleanup use of backend name

- Use the backend name as the `name` in all the BackendMessages associated
  with the message tester, so we can view just those and easily clear them.
- Don't pass backend names in URLs and methods - just use a fixed backend
  name for HTTP tester, 'message_tester'
Refs #331: Reduce use of LoggerMixin

It turns out most of our remaining uses of LoggerMixin are in
base classes that apps are expected to extend, so we probably
shouldn't take that out until the deprecation period is over.

I did add a bit to the release notes about this, to point out
that code that extends those classes might be using LoggerMixin
indirectly.
Refs #343: Start a table of the different routers' characteristics
Merge pull request #331 from rapidsms/331-dont-use-loggermixin

Don't use deprecated LoggerMixin
Refs #332: Add doc for database backend
Refs #349: Customize database router batch size
Refs #332: Allow importing rapidsms.backends.db.DatabaseBackend
Refs #349: Test with one message, many connections
Refs #332: Add note about db backend

Add note suggested by copelco about the db backend not likely
to be used in production.

Write your commit message in the imperative: "Fix bug" and not "Fixed bug"
or "Fixes bug."  This convention matches up with commit messages generated
by commands like git merge and git revert.
Merge pull request #349 from rapidsms/feature/349-customize-dbrouter-chunk-size

Allow DatabaseRouter chunk size customization
Merge pull request #332 from rapidsms/doc/332-database-backend

Add doc for database backend
Refs #343: updates from code review

@copelco ready to review again
Refs #328: remove internal fields from message tester table display

@copelco ready to review again
`python manage.py` not `manage.py`
Merge remote-tracking branch 'remotes/origin/feature/bulk-messaging-api' into feature/328-httptester-use-db-backend

Conflicts:
	docs/releases/0.13.0.rst
	rapidsms/backends/db/__init__.py
Rename database backend from `db` to `database`

This is because of a Django app name collision with the database router.
Merge pull request #343 from rapidsms/docs/343-router-table

Add table of router features to router/index.rst
Use full name of database backend instead of DB backend.
Merge pull request #328 from rapidsms/feature/328-httptester-use-db-backend

Potentially switch httptester backend to new db backend