Added sortable option

"sortable : true" option can be mentioned in dropZone  . The index value
to sort will be passed to drop  function
Added handle option

Can specify as option in 'dragZone'. If specified, restricts dragging
from starting unless the mousedown occurs on the specified element, Only
elements that descend from the draggable element are permitted.
If sortable prevent drop without index
Updated readme

added sortable example