add DragDropOrderedModelAdmin

[shuckc]

This is the code from #35 as posted on https://gist.github.com/devsnd/38b9d4ff573a0611a87f10ab3042db45 with a couple of changes to work on master.

It could use a few features but essentially it works.

• indicating the drop target
• some sort of icon or row decoration to indicate it can be dragged
• tests and documentation

[shuckc]

I think it would be ideal if we added this support to the existing admins (list, inline-stackable and inline-tabular) and configured it by using a different field/column name. This would allow move buttons, drag-and-drop or both.

[lsmith77]

any chance we could get this finalized?