bes/django.py: Prefix configs with BULK_ELASTIC_SEARCH_LOGGING_*

To match 'bes', the name of our package, and avoid ambiguity with
other Elastic Search configs.
test/test_django.py: Add ConfigTestCase.test_django_config_override_default

Make sure our config-overrides work as expected.
README: Flesh out with usage examples

The link to Elastic Search's 'index' is off by one, because there are
two 'index' ids on the glossary page.  I've filed a bug [1], but until
it's fixed just link to the preceding entry.

[1]: https://github.com/elasticsearch/elasticsearch.github.com/issues/511
README.rst: Add symlink for GitHub rendering