take advantage of Sprockets

If Sprockets is unavailable (Rails 3.0), we continue to copy CSS, JS,
and images over to [HOST_APP_DIR]/public/. If Sprockets *is* available,
however, we take advantage of it by putting the assets in app/assets/ and
letting the asset pipeline pick them up.
Merge pull request #53 from jamesarosen/sprockets

take advantage of Sprockets