Fix parsing of Expiration. Fix comparison of Expiration. Add unit tests.

[skst]

This PR fixes #17.

• The Expiration property was returning an Unspecified DateTime type. Added the AdjustToUniversal style to Parse.
• Convert DateTime passed to ExpirationDate to UTC before comparing with Expiration property (which is UTC).
• Added a validation to Expiration.set to ensure the incoming type is not Unspecified.
• Added a validation to ExpiresAt to ensure the incoming type is not Unspecified.
• Added more unit tests around ExpiresAt and Expiration.
• All unit tests pass.

This is my first pull request ever. 🙂 I tried to add relevant tests and to maintain the project's coding style (indents, spaces not tabs, etc.). If I introduced an error or missed something, please let me know, and I will be happy to correct it.

[skst]

I have updated the pull request to ensure time-zone-agnostic comparisons with the expiration date. The date/time passed to Expiration and ExpiresAt is treated as a date only--the time and time zone are ignored. ExpirationDate similarly ignores the passed time and time zone and uses only the date component to compare with the stored expiry date. This way, a license always expires at midnight local time. Granted, this may give a user a few hours less or more of usage, but it's consistent with user expectations.