Better MCV pattern

+ "on-drag" class added or removed instead of hard-coded opacity for
elements being dragged (opacity moved to css file)

+ jQ-lite addClass and removeClass instead of pure js string
concatenation (makes jQuery addClass break)

+ when-drop function returns the target element as a second optional
parameter
README update
Revert "README update"

This reverts commit 3e726025b1d96a21a0af3273de2b1bd6b555f245.
Merge pull request #3 from floribon/master

Better MCV pattern