Testsuite support

Couple of commits to add "--testsuite" phpunit's option into this library.
Found a bit annoying having to specify the paths where to look for the tests when they were already defined in the phpunit's configuration file.

This implementation looks up into the configuration file for a given testsuite, parses its defined directories and adds them into the paths to look up for tests to run.
I've just added the "<directory>" tags from the config file, but it could also do with adding the "<file>" tags.

I'd like to say that I'm no Python developer, so apologizes for the more than probable wrong written code. It just works with my scenarios. Please feel free to edit/change or make it more robust as you see fit!