Moved ESTestCase from `elasticutils/tests/__init__.py` to `elasticutils/estestcase.py` because it makes it usable outside of our stuff.

It's interesting to note that we have two different ESTestCase classes: one here and one in `elasticutils/contrib/django/estestcase.py`. I tossed around unifying them more and then decided I didn't want to do that for 0.9. I'm not really sure what to do about that--seems weird to have two different ones.