Add Subsonic API

Add Subsonic API (http://www.subsonic.org/pages/api.jsp).
This make Ampache compatible with all Subsonic clients transparently :).
Tested with Periscope and Subsonic Music Streamer on Android.

This API use XML/RPC ACL. To test by yourself, just merge your local repository and be sure XML/RPC api is available for your user/ip address. Then logon on your Subsonic client with your ampache installation url as Subsonic server address, and with your Ampache login/password.

Not supported Subsonic features (optional for most clients):
 - Stars (data doesn't exist, only rating)
 - Scrobble (it's part of Ampache)
 - Shares (data doesn't exist)
 - Podcasts (data doesn't exist)
 - Jukebox
 - Internet radio
 - Chat messages (data doesn't exist)
 - User management (we have to care about Subsonic roles mapping)
 - Bookmarks (data doesn't exist)