Remove some code which was never covered anyway, add note about implementation.
Turn runtime errors into asserts (improves coverage).
Another test, and coverage update, for invalid options.
Mark OrderedDict import for no cover.
Simplify _enum_count implementation.
Fix picklability of cache.
Make _enum_count regular dict (set_enum_count is always called anyway, simplifies implementation).
Merge pull request #24 from mcmtroffaes/feature/full_coverage

Full test coverage.