fix(draggable): bind drag events to $document

$body won't receive mouseup if the cursor is outside the browser window, $document does
fix(gridster): Removing previously added style tags before adding new one.

Hopefully fixes #211 and #294.
fix(draggable): fix offset during drag

when the scrollX/Y is changed during drag (e.g. by the mousewheel)
the offset wasn't correct
fix(draggable): replaced scrollX/Y with scrollLeft/Top

window.scrollX/Y is only available in Chrome, not in IE
fix(gridster): fix add_widget to use correct size_y when adding rows
fix(draggable): handle both touch and click events

Fixes #207, #236, #329, and #380
Release 0.5.2