Asserts

Added

- Add a stub file.
- Add assert_count_equal().

Added

- Add assert_less(), assert_less_equal(), assert_greater(), and
assert_greater_equal().
- Add assert_not_is_instance().

Changed

- assert_datetime_about_now()/assert_datetime_about_now_utc(): Handle
comparison with None more gracefully.

Added

- Add the LICENSE file to the distribution.

Initial release.