Fix parsing of the 'content' property.

Conflicts:
	src/components/style/properties.rs.mako
CSS parsing/selector-matching for pseudo elements
Change range to iter() and add comment for FirstLine and FirstLetter
auto merge of #1464 : parkjaeman/servo/pseudo-element-selectors-matching, r=SimonSapin

CSS parsing and selector matching for pseudo element :before and :after