rename database module; bugfixes; etc.

* rename database2.py to database.py
* move some constants into settings; update util.load_settings()
* remove some unused things from util
* update webserver to allow auto-reloading
* update webserver.TrackInfoNamespace to work properly when webserver
runs without a main program console
update README.md
rename database as library; new playlist module

also update READMEs
playlist: /current; force song_ids to be valid

also:
* add current marker to /playlist
* new `errors` wrapper to translate exceptions into `abort` calls for
endpoints
* add library.Song.exists(id) function
* fix 'dict_facotry' typo in library and playlist
* add functionality to local jsonify function to control whether it
aborts on data==None, and what response code it uses if it does