Add support for terms and wildcard queries
Document filter with None value
Fix ESTestCase and 501/503 templates for middleware

ESTestCase now derives from django.test.TestCase which means
it requires DATABASES settings. This adds sqlite in-memory
settings to test_settings.py.

Moved the middleware error templates so that they're in
the elasticutils.contrib.django Django app.
Fix issues in the CHANGELOG
Fix index_objects

The problem was that the number of objects you could index with
index_objects was tied to the number of things you bulk index at
once. That was certainly problematic.

This tweaks things so they're not connected and provides a
reasonable-ish default.