Pypi fixes

Changes to get the Python bindings into PyPI:

* Fix versioning
* Update my e-mail to the non-Google one.
* Customize README for Python usage
* Fix build artifacts and package libgumbo.so with the distribution.
* Be resilient to the absence of html5lib or BeautifulSoup on the target machine
* Support Python 3

It's still probably a bit fragile on non-Linux platforms; I lack machines to test installation there, so I'll have to wait for the bug reports and pull requests.