Include non-writable db backend

[jayvdb]

One quite simple backend is a non-writable backend, which supports creation, allows models instances to be instantiated with an in-memory counter per model, and errors for any write operation.

This would be used only for tests which do not need any writes, effectively even faster than https://github.com/koterpillar/fast-test-database and saner/more-controllable than https://github.com/liumengjun/django-fast-test as it could be mixed-and-matched with other databases for tests which do need to perform writes.

The intention would be to use it as a fallback database for idlesign/pytest-djangoapp#7 , if the test config indicated no writes were needed.