Add an ack on all stream_listener transfers; Bump to 0.2.8

* Slave: Add a `stream_listener_ok` ack for all listeners sent during a
  handoff. Sending process waits for all acks to come in before exiting.
* Preroller: Short-circuit if the socket has already been destroyed
* Master: When a slave disconnects, remove its listener counts
* Shoutcast & RawAudio: try to catch socket errors and call disconnect
* Slave: Send listener counts every 10 seconds, not every 30