Turns comma splitter into a more generic splitter.

* No longer accepts splitted value as constructor argument but in split method instead.
* Accepts separator value to split over in constructor.
Fixes #316 - incorrect background processing.

* Reworks advanced processing for backgrounds so all properties are correctly identified.
* Fixes removing `0 0` background-position as it's default.
Adds `background` normalization according to W3C spec.