add Scroller.hx : scrolling image with custom direction and custom speed
Fix issue #31 and update Scroller

issue #31 fixed : in DisplayObjectContainer, move the dispatching of
ADDED at the end of the add function

Update Scroller to simplify its usage : no more call to init needed
(init is now a private function)
Merge pull request #32 from Ynk33/master

add Scroller.hx : scrolling image with custom direction and custom speed