[Piglit] Refactor summary code

[Dylan Baker]

This short series refactors some of the ugliest code in summary.py,
mostly code that deals with html generation. The resulting code is
simpler and cleaner.

This moves some logic out of the mako template into the python
generator, allowing the logic itself to be simplified.

Finally, this itroduces a new module for working with piglit groups,
which is mainly a wrapper around posixpath, but should be clearer in
purpose and gives us the ability to silently change the internal
representations of groups.

This is available at my github:
https://github.com/dcbaker/piglit submit/summary-cleanup