fix: prevent loader.stop() from hanging

This fix addresses issues found here: https://groups.google.com/forum/#!topic/kivy-users/wAPRCk9eHYg

Just a comment here. This change would alter the current "stop" behavior, but for the better in my opinion. The stop method currently does not "stop" any loading, but waits until all tasks have finished. That seem counter to what I would have guessed.

If there is an objection to the change though, I could re-submit the PR adding a "clear" or "reset" method?. It certainly seems to solve the locking issue found above in a simple, clean and robust way?

ps. Resubmitting so it can be auto-merged