Added StyleSheet.isEmpty/nonEmpty checks for the 'empty' state of a StyleSheet.
Added convenience HTMLTag.style(PseudoClass) to create a selector on the tag + pseudo class.
Updated Spaces.spaces to be a ListMap instead of Map to retain the order.
Updated StyleSpaces to only render StyleSheets that have attributes.
Created ExternalStyle Module to use external (CSS in the head of the document) styles instead of inline.
Removed HeadStyle as it was broken and is now replaced by ExternalStyle.