Fixed Date documentation

Small errors in doc were corrected
Added test for boolean truth
Improved repr test, fixed bool test in iso
Completely revamped Date tests

- Made clearer operator and comparison tests (e.g. using operator module instead of lambdas)
- Renumbered tests according to established scheme
- Improved hash_equality, bool, repr, str, pickling tests
- Added create_from_attr, subclassing tests