add on_transform_with_touch() callback so classes derived from Scatter have a way of detecting and reacting to touch transformations without re-implementing the entire transform_with_touch() function
Change on_transform_with_touch to be a proper kivy event instead of just regular python method
Merge branch 'scatter-on_transform_with_touch' of git://github.com/bob-the-hamster/kivy into bob-the-hamster-scatter-on_transform_with_touch
scatter: add versionadded