Fix setting _p_changed when removing from small Python BTrees/TreeSets.

Pure Python Set objects were also not marking themselves changed on addition. This impacted things like zope.index.topic.filter

Make all the tests in the w_zodb Tox environment run.

Fix setting _p_changed for small pure-Python BTrees.

Fixes #9 (I hope).

[ci skip]

Lost 56bd1d04 in a merge?

[ci skip]

Features added, new Python versions / platforms supported.

[ci skip]

Using 'random' in a test exercising a known bug is a code stench.

See: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

They are always empty, except for docstrings, but the metaclass confuses
the coverage bits for branch detection.