Fixes #325 - removing invalid special declarations.
Fixes #323 - variable references being stripped out.

* Advanced optimizations haven't supported CSS variables.
Improves performance of advanced mode validators.
Fixes #307 - background-color in muliple backgrounds.

Apparently multiple backgrounds allow background-color in last value only:
http://www.w3.org/TR/css3-background/#the-background
Fixes #322 - adds background-size support to advanced optimizer.
Version 2.2.10.