onStartMoving() fix to minHeight
onStartMoving() fix to minHeight

*method was not correctly counting margin when figuring out minHeight during a drag
(see #999)

Note: many other places don't handle margin, both height and width of cells.
Should probbaly be fix globally. WidthMargin is not
in opts though but CSS, making it harder.

radiolips/gridstack.js,radiolips/gridstack.js,troolee/gridstack.js