This is a utility class that allows anything mixing it in to notify a set
of listeners about interesting events.
Adds the passed proc as a listener on the channel indicated by
channel_name. listener_key is used to remove the listener later; if none is
specified, the proc itself is used.
Whatever is used as the listener_key is returned, making it very easy to
use the proc itself as the listener_key:
Removes the listener indicated by listener_key from the channel indicated
by channel_name. Returns the registered proc, or nil if none was found.
Calls all the procs registered on the channel indicated by channel_name. If
value is specified, it is passed in to the procs, otherwise they are called
with no arguments.