Draggable revert: option can be a callback

Attached is a patch w/ test that adds the capability for draggable's revert: option to be a function, rather than just true/false/(in)valid. The function is called on drop, and the return value is used the same as constant true/false values. It gives more flexibility than simply using a constant value.

Caveat: I'm not sure what arguments should be passed to the callback, if any - I couldn't tell where the standard (e, ui) come from, or if those are appropriate.

(I hope I'm doing this right. Sorry, it's my first ticket/contribution here. And I'm not sure how the test suite works, so mine's a separate file. Thanks!)