Updated StyleSheet to contain a single Selector instead of a List[Selector].
Created HTMLPage trait that extends from Page to provide references for html, head, and body.
Added HTMLTag.page to get a reference to the current HTMLPage.
Began work on HTMLTag.allStyles to return all referenced styles for a tag.
Fixed Selector.matches for AttributeExistsSelector and AttributeSelector.
Finished HTMLTag.allStyles.