much faster scanning, more reliable XML, better id3 browsing, fix transcoding bugs

new dependency though: flask-ext-sqlalchemy http://pythonhosted.org/Flask-SQLAlchemy/

biggest issues fixed are:
zombies when transcoding with external transcoder and closed connections (were crashing my system / memory leaking)
speed of scanning is vastly vastly improved (using 'materialized paths' for the database instead of folder id and parent id which also improves the speed of many other functions)
better, more reliable metadata scanning using the MediaFile library from https://github.com/sampsyo/beets

Looking for comments on the changes and feedback.  If you'd like me to separate some of these changes in to individual pull requests, etc then just let me know.