Fixes #218 - gets rid of @import statements appearing after CSS content.

See www.w3.org/TR/CSS21/cascade.html#at-import for details.
Fixes #220 - universal selector between two comments.

Comments were incorrectly processed when universal selector was placed in between.
Fixes #217 - removing extra whitespace from attribute selectors and urls.

Browsers strip the whitespace before evaluating so should we.