Add an 'exclude' option, fixing #31

As promised, I implemented the ``exclude`` option using ``shutil.copytree``'s ``ignore`` callback.
Ah, I see. That's completely valid, and defining our own callback is simpler than trying to force `ignore_patterns` to do what we want.

So if you want to make the simple fix from my other comment, I'll give this one more look through and then merge it.