Add django tests

This fixes MappingType.get_index() and also reworks the django tests so they're in their own module and broken up across several files and use elasticutils.contrib.django.estestcase.ElasticSearchTestCase because using that:

1. reduces the likelihood that it's broken
2. uses the thing we wrote for making testing elasticsearch things in Django easier to test elasticsearch things in Django -- that seems like a good idea

r?