Set PURE_PYTHON=1 for coverage testing under tox.

A Python-implementation-specific test for the repeated-search-optimization gets us to 100% coverage.

Also fixes a few discovered differences between the C and Python implementations.

Add a test and a fix for the __of__ issue @davisagli reported. Only tested under Python2 because I couldn't figure out how to reproduce it under Python 3

Adapt the garbage collection tests to run under PyPy. This makes the overall test diff smaller and easier to read.

Fix the last doctest errors in pure-python mode about parent circles. They were another manifestation of zopefoundation/ExtensionClass#3 and required a workaround.

Add tests for all the methods proxied by the C wrappers, and make the Python implementation match that behaviour.

All tests continue to pass under 2.6 and 2.7.

The pure-python code currently raises AttributeError instead of RuntimeError in the
recursive-parent cases, though I'm not sure why that is. There may also be some issues
with the proxying of some methods, but those aren't covered by current test cases.

setup.py: Set url to github

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

Only in ASCII encoding.

Fixes LP #143358.

README.rst in root, w/ prolog.

CHANGES.rst in foot.