Improve date asserts

[nunofgs]

This PR:

• Improves the date assert by validating using the moment API. This now allows the following assert to work:

new Assert().Date({ format: 'MM/YYYY' }).validate('11/2016');

• Adds optional format to NullOrDate assert.
• Improves date validation in the date diff asserts by using moment's parsing.
• Fixes a typo in the README.

Note: due to the changes in the way dates are handled, this PR could constitute a BC and releasing a major bump is recommended.