Add a 'described' method so listeners can get proper notification
of description changes, and update their description in between
progress changes.
Add a MultiCombinedTask that reports on progress over all tasks
combined.
* examples/trm.py:
Add a playlist option to the example. Still needs to store results
to pickles, preferably after each completed task.
* morituri/common/checksum.py:
Add audioconvert to make sure we can trm ogg files.
Add a task to calculate a trm fingerprint.
* examples/trm.py (added):
Add an example.
Strangely enough it starts burstily, doing 10% directly, halting,
then progressing quickly,
