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 $redis object and
    not the internal connection. I think that is the correct behavior.
We are using 14 as the test database
Can now subscribe to blpop/brpop as many times as you like
Reverse on(blpop) result: ($error, $data, $list_name)
Documented on({blpop,brpop} => ...)
Merge pull request #25 from marcusramberg/feature/24-on-blpop

Add support for $redis->on({brpop,blpop} => @list_names => $cb);