Add a form sheet manager singleton to use subclass in storyboards

Because the [`presentFormSheetWithViewController:animated:transitionStyle:completionHandler:`](https://github.com/m1entus/MZFormSheetController/blob/master/MZFormSheetController/MZFormSheetController.m#L930) method wasn't able to use the subclass I've created to support custom animations.

And, by the way it could be a way to handle further configurations that cannot be achieve with appearance selectors... What do you think ?