bind drag events to $document

$body won't receive mouseup if the cursor is outside the browser window, $document does

Steps to repeat:
* open http://gridster.net/demos/resize.html
* resize the browser window so it isn't in fullscreen mode
* drag a widget and move the cursor outside the window
* release the mouse button
* move the cursor back inside the window
* result: the widget is still dragging until you click

Fix:
bind the mouse-events to the document so it will receive the mouseup even if the cursor is outside the browser window - this creates a more native drag-n-drop feeling.