move images to where sprockets expects them to be

Sprockets expects images in an engine to be in an [engine name]
directory, but the jquery ui images weren't.

The jquery ui stylesheet had to change, too, to reference images
starting at "/" rather than relative URL's.
fix code that figures out if the asset pipeline is enabled

The Rails3-compatible algorithm to figure out if the asset pipeline is
doesn't work with Rails4.  Quite frankly, the new one doesn't seem as
clean as the old one, but hopefully this code will work in all cases.

https://github.com/rails/rails/issues/10334

Note that the specs didn't seem to test both cases, or else the image
path is now the same whether the pipeline is enabled or not, but in
any case the specs don't need to determine whether the pipeline is
enabled, they can use the same path in all cases.