Undocumented support for allowing @import to work on .css files.

Mostly for the benefit of django-pyscss (which thusfar has explicitly
forced this behavior) so it doesn't need to copy and paste large blocks
of fiddly code, but seems like a popular feature request in Ruby Sass as
well.
URLs with quotes should produce Url, not Function.  Fixes #312.
Restore a missing `return`.
Comment update.