Incompabilities with MPD protocol v0.19

[kingosticks]

It looks like we missed a few changes in version 0.19 of the MPD protocol in https://github.com/mopidy/mopidy/pull/1213. Below is the entire list of protocol changes taken from MPD's 0.19 (2014/10/10) changelog:

• [x] new commands "addtagid", "cleartagid", "listfiles", "listmounts", "listneighbors", "mount", "rangeid", "unmount"
• [ ] "lsinfo" and "readcomments" allowed for remote files
• [x] "listneighbors" lists file servers on the local network
• [x] "playlistadd" supports file:///
• [ ] "idle" with unrecognized event name fails
• [ ] "list" on album artist falls back to the artist tag
• [ ] "list" and "count" allow grouping
• [ ] new "search"/"find" filter "modified-since"
• [ ] "seek*" allows fractional position
• [ ] close connection after syntax error

The new grouping functionality for list is being used by MPDroid and was originally reported in https://github.com/mopidy/mopidy/issues/1314.

• [ ] sticker database