handle css @import spidering

[zapnap]

Discussed in IRC the other day. Noting it here for posterity. Could look into using http://github.com/alexdunae/css_parser for this, although there may be a more efficient path.

parser = CssParser::Parser.new
parser.load_uri!(uri)
parser.loaded_uris
=> [uri, imported_uri_1, imported_uri_2, etc]