Add support for Python 3.6

[jamadden]

• Switch to using zope-testrunner instead of setup.py test because of the namespace package issues.
• Enable pip caching on travis.
• Stop testing pypy3 on travis since it implements Python 3.2 and is not supported by pip anymore. There is a 3.3-alpha available on travis, but that's a dead end and not developed anymore since 3.5-beta is out (but not available on Travis AFAIK).
• Enable coverage reporting; currently at 100%.

Fixes #6.

[jamadden]

Thanks!