Added draggable support

Added a basic dragging support for the group element.
( based of this site's code:
http://wesleytodd.com/2013/4/drag-n-drop-in-raphael-js.html )
Draggable now pays attention to initial transform values
Made scale with separate axis optional

If you write group.scale(3), it makes translate="scale(3 undefined)",
fixed that.
Merge pull request #3 from meszaros-lajos-gyorgy/master

Added draggable support