AUI-1360 Make aui-sortable-layout keyboard accessible

Hey @marclundgren,

Here is what I have so far for making the drag/drop keyboard accessible in the sortable-layout module. Can you please take a look and see if everything looks good?

Keyboard usage:
Use tab key to focus the first draggable element and then use the left and right arrow keys to navigate through all the draggable nodes. Hit ENTER to select an draggable object to drag. Then use the left and right arrow keys to select a drop target. A drop target is only valid when the blue placeholder appears. 

Issues:
- Right now, you can only place a dragged node after a drop target. I'm going to work on adding keys to allow user to select the quadrant for the drop target, so that a dragging object can be placed before or after the drop target.
- I'm setting the lazyStart attribute to false for now in the demos index.html. The lazyStart attribute is causing  a delay in the placeholder on showing up in the beginning. You have to press the arrow keys several times for the blue line to appear when the attribute is set to true. I'm not really sure what this attribute is for, but I'm wondering if I can just hard coded the lazyStart attribute to always be false when the keyboard is used.

Thanks!