Add the ability to move items across different list

v1.3.0

Dragging across lists/groups

Determine drop location based on mouse direction

The logic for sorting is no longer based on the original order, but based on the mouse move direction.
If the mouse is moving up item will be dropped above the drop target. If mouse is moving downwards, it will be dropped below the drop target.

Additionally, the drag image is set and on Edge where setDragImage is not supported, a workaround is implemented.
LIMITATIONS: Drag image does not work well in IE11

Global functions were moved into the Sortable-list element for better access to the list from the drag* events.

IE bug fix

IE does only allows "text" and "url" as the first argument to dataTransfer.setData and dataTransfer.getData

Polymer 2 and drop target bug fix

Hybrid conversion to work in Polymer 1.11 and Polymer 2.6.
Fix drop target sorting issue (see afloesch#4).
Augment component to accept a handle-selector which specifies draggable element.