Second pull request

Couldn't wait for the docstrings, I'm nearly completed with my additions. The ``List`` instances are now able to output an EDL compatible string with their ``to_string()`` methods (also added ``Event.to_string()`` and ``Timewarp.to_string()`` etc.).

Fixed a couple of other bugs and added a couple of new methods and one new class. All my additions are done with tests prior to implementation.

Please read my commit messages. The only missing thing is the audio track information, but I'll leave it like that for a while, cause with its current form the package seems suitable to be used in our pipeline.