Add Curve loading to hooke.playlist.Playlist.

Add Curve loading to hooke.playlist.Playlist.

Highlights:
  * NotRecognized.__get/setstate__ for Pickling through the Queues.
    Perhaps this would be better solved in Failure.__get/setstate__?
    The current fix works for the moment though.
  * Use Curve.data == None to indicate unloaded curves.
  * Fix TutorialDriver.is_me(self) -> is_me(self, path)
  * Added ._load* to Playlist, so it only caches ._max_loaded curves
    in memory.
  * Fix broken Playlist.from_string doctest.