Formalize type for nth expressions.

Need something to return both the coefficient and the constant.
Fix class selector when multiple classes set.
Clean up selector tests.
Consistently use Selectors spec's definition of whitespace.

Note many parts of the code still use NSCharacterSet's
whitespaceAndNewlineCharacterSet. These are the areas where I don't yet
particularly care about exactly following the spec.
Implement attribute selectors.