Add the OutputLilyPond Directory to .gitignore

Users should always copy an updated version of OutputLilyPond themselves.
Now It Starts!
Porting Improvements from vis9b (Importer)

- Removed GUI option for multiprocessing (we'll do it by default)
- Enabled multiprocessing import of MIDI files
- Enabled importing music21.stream.Corpus objects
- Added "import_running()" and supporting stuff, so in the future we can
  cancel imports
- Rewrote Importer.run() for always-multiprocessing, except with MIDI
More Changes from vis9b for Importer

- add_files() taken from 9b
- add_directory() simply calls add_files()
Hooked Up the Importer to the Progress Bar

Somehow, the GUI still isn't updated correctly. Hope we don't have to go
back to that clumsy "ImporterThread"-style thing we had in vis9b.