mpd protocol

It is in a very early stage, but I started working on the support for the mpd protocol (see #51).

By now forked-daapd excepts connections from mpd clients (on port 8800) and returns static responses for "status" and "currentsong". For all other commands it returns an error.

@ejurgensen it would be great if you could have a look at the approach i have taken. Any feedback welcome!