Add a 'done' callback

Add 'done' callback for when dragging is complete
Move check for background-size: cover inline

I had an issue where I was setting the size after image load and checking immediately was failing for me.
Add mouseleave event for unbinding drag

When dragging background and mouse leaves the outer div
Merge pull request #10 from jeffsaracco/master

Fix bug with background cover - add mouseout - add done callback