Treats Expires_in and x_refresh_expires_in as elapsed time in seconds.

[ryan-sandy]

This PR fixes an inconsistency with timestamps within the project.

The current code treated the expires_in and x_refresh_expires_in as timestamps if they were passed in as constructor parameters. If a token was set using the setToken the expires_in and x_refresh_expires_in became timestamps from Date.now(). This lead to inconsistent behavior in the validation methods depending on how you created the token.

To handle stored tokens, an optional createdAt parameter was created. This stored tokens to have the correct expiration date, instead of using Date.now().