Transcode unsupported audio formats to Ogg

Transcoding is handled on the backend with avconv from libav.

The client requires songs via a new endpoint:
    GET /song/<id>/<wanted_formats>
where id is the integer id of the song in the database,
and wanted_formats is a comma-separated string of file extensions,
e.g.:
    GET /song/491/ogg,mp3,m4a

If wanted_formats includes the file's original format, it's sent
without transcoding.

Otherwise, if wanted_formats include ogg, it's transcoded to ogg.
No caching is done yet.
Reorganize readme
Remove stray debugging statement
Route album art requests through new endpoint

This obviates the need to ever send filenames to the client side,
and also gets rid of an extra loop in searching.
remove unused hybrid property
Remove unused defaults