Added content-type header usage to icecast sources and clients

Mountpoints should now properly send the correct content-type along
when a client requests it.

Split off the mp3-specific icecast loop into a separate method

Fixed nil-pointer dereference on a racey metadata update

Added more buffer space on the MultiWriter pending channel, this
improves client-acceptance rate.
Fixed missing return in one of the error branches

Documented the "race" between the detector and handler