Add new flag to make centering plugin while dragging optional

Currently, the plugin will disregard the location at which the user clicked the box, and centers the box around the mouse. For example: If I click and drag the top left of the box, as soon as I move the box, it will jump so that my mouse is in the center of the box.
New flag example: If I click and drag the top left of a box, the box will stay where it is relative to my mouse.

Note: You may want to re-compile and minify the coffee script as I used web based tools to do so, as I don't normally use coffeescript.