Move #expand_properties to StyleSheet
Move abstraction resolution to it’s own class StyleSheetAbstractionResolver
Move lookup of abstract properties to StyleSheetAbstractionResolver
Move StyleSheet::Context to StyleSheetContext
Reword abstractions as abstract_rules

This change in language also lead to a slight refactor/renaming of StyleSheetAbstractRulesResolver (previously called StyleSheetAbstractionsResolver)
Restructure code under Style namespace
Remove unused instance var
Update style sheet spec to use hash matchers rather than #include
Introduce Style::Rule to encapsulate rule related logic
Introduce concise extension syntax (with examples)
Reword examples and specs to use typography instead of type so as not to confuse matters