Extended on() to support "blpop" and "brpop"

  Questions:

  * Currently on() will die if already listening to the same
    "blpop list_name". Not sure if that is right.

  * Should unsubscribe() be:
    $redis->unsubscribe(blpop => "list_name", $cb);
    or
    $redis->unsubscribe(blpop => "list_name"); # current implementation

  * Currently the callbacks is called with the main $redis object and
    not the internal connection. I think that is the correct behavior.

See also #24.