music streaming server / subsonic server API implementation

features
*  browsing by folder (keeping your full tree intact)
*  browsing by tags (using taglib - supports mp3, opus, flac, ape, m4a, wav,
   etc.)
*  on-the-fly audio transcoding and caching (requires ffmpeg) (thank you spijet)
*  pretty fast scanning (with my library of ~27k tracks, initial scan takes
   about 10m, and about 5s after incrementally)
*  multiple users, each with their own transcoding preferences, playlists, top
   tracks, top artists, etc.
*  last.fm scrobbling
*  artist similarities and biographies from the last.fm api
*  a web interface for configuration (set up last.fm, manage users, start scans,
   etc.)
*  support for the album-artist tag, to not clutter your artist list with
   compilation album appearances
*  written in go, so lightweight and suitable for a raspberry pi, etc.
*  newer salt and token auth
*  tested on dsub, jamstash, sublime music, and soundwaves