The drag speed can be changed now

This is useful, if you have a draggable <g> inside a scaled <g>. If the
outer <g> is scaled by 2, then the draggable <g> moves twice the amount,
so the drag speed needs to be set to 1/scale.

Also, minor cosmetics on the code.
Merge pull request #4 from meszaros-lajos-gyorgy/master

The drag speed can be changed now