Use a configurable list of views as an alternative to @db_write

Fixes #7. There is now a ``MULTIDB_PINNING_VIEWS`` setting in which views are listed. like this:

```python
MULTIDB_PINNING_VIEWS = ('myapp.core.views.myview',
                         'anotherapp.contrib.greatapp.views.yourview')
```

The views listed pin the thread.

Btw, how am I supposed to run the tests for ``multidb``? Because I'm doing it in an odd way (putting ``multidb`` in the ``INSTALLED_APPS`` of a Django application and running ``./manage.py test multidb``).