Don't use/poison global wheel cache when building w/ PURE_PYTHON.

[ci skip].

Drop support for Python 2.6 and 3.2.

- 2.6 is long out-of-maintenance, and a security quagmire.

- 3.2 cannot be tested on Travis.

[ci skip]

[ci skip]

Fixes #27.

Test plan:

- run detox to generate ALL THE WEIRD FILES
- run check-manifest to see if any made it into the sdist

Support for ZODB on PyPy

Remove aliasing of PickleCache's target_size/cache_size; just use cache_size as specified in the interface.

Replace all remaining uses of 'self.__X' in persistence.py with _OGA/OSA for consistency; also avoid some duplicate calls through __getattribute__ which were more obvious in the OGA form.

Remove superflous comment about interning of unicode dict keys on Python2. The existing behaviour matches the C implementation.

Use _p_deactivate unconditionally in test_del_jar_of_inactive_object. I'm not sure why I was reaching into the internals of the Python implementation.

Remove mention of Jython, although not the supporting bits. Hopefully tox/virtualenv/pip come together soon to allow official testing and mention.

Add an explicit assertion that the status of an object is no longer a ghost after accessing _p_mtime...that the value of _p_mtime is set is implicitly confiramtion of this because it gets set during loading, but explicit is better than implicit.

Make the Python impl match the C impl by automatically unghostifying the object when _p_mtime is accessed.