Rename Drag[abble/Manager] to Sprite[Manager], add ondrop() etc.

--HG--
rename : src/yoob/drag-manager.js => src/yoob/sprite-manager.js
Add onpickup(), moveToFront(), moveToBack(), getSpriteAt().
Fix name of example file.

--HG--
rename : eg/drag-manager.html => eg/sprite-manager.html
Add .onclick event to Sprites, .onpickup->.ongrab, and such.