Remove getParameter method of StyleDict

Fold it into its only caller.
Rename CPSKeyNotFoundError to CPSKeyError

For consistency with KeyError.

Also rename errors.CPSKeyNotFound to errors.CPSKey for the same reason.
Improve some comments
Add FailoverStyleDict

Add FailoverStyleDict which looks up a parameter in multiple StyleDicts,
plus tests.

Add a base class _StyleDict from which both FailoverStyleDict and
StyleDict derive.
Merge pull request #182 from rrthomas/master

Add FailoverStyleDict, plus various improvements to StyleDict