Clean up, correct and reduce documentation

Fixes #209
Add elasticutils.estestcase.ESTestCase

This makes the ESTestCase "public". Previously, it was intertwined with
our test suite.

Further, it adds documentation for it to the api.

In the process of doing that, I nixed a bunch of skip_tests stuff. If
someone doesn't want it to run tests, they should wrap the class on
their own. That's a lot easier than us providing scaffolding that is
either generic (hard) or requires a specific test framework
(pain-in-the-ass-for-users).

Fixes #215
Update CHANGELOG

* add note about ESTestCase