Refactor message handling to be more flexible

New parameter `handlers` has a list of message types for which
corresponding handlers should be called. For example, for message types
['delete', 'limit', 'disconnect'] the handlers on_delete, on_limit,
on_disconnect will be invoked if the streamed message has the
corresponding attribute. If the handler returns True then more handlers
might be invoked, otherwise message processing stops. Before all
handlers are called, special on_success handler is invoked with the same
logic: continue message handling if True is returned.

This approach maintains backwards compatibility by not breaking any
existing behaviors.
Combine two ifs into a single line
Merge pull request #225 from extesy/master

Refactor message handling in TwythonStreamer