Adapt to new structure introduced in jQuery UI 1.11.0

I gave it a try. It builds the assets and they seem to be correctly generated. But I didn't actually test the generated gem yet, while I intend to do it tomorrow. Anyway, there are lots of incompatible changes proposed in this PR to consider.
Sorry, will fix that. I haven't noticed the testapp directory before. I'll try to figure out how this is intended to be used...