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.

Merge branch 'zodb-on-pypy-support' of https://github.com/NextThought/persistent into zodb-on-pypy-support

Add a test checking the non-cooperative behaviour of Persistent.__getattribute__ in both C and Python.

Another CPython/pure-python difference is the handling of interning instance keys. C has always let non-str values through, but the python implementation raised a TypeError. Correct that and add a test case.

Remove the unused __ring_handle, and tweak move_to_head to avoid recursive call to __getattribute__ based on benchmarks.

Add a non-CFFI implementation of the Ring abstraction based on the deque work, and add a tox environment to test it. Only one ZODB test fails.