Improving ISO Date handling

 * Using a new library ([segmentio/isodate](http://github.com/segmentio/isodate)) to handle date string parsing (instead of our custom hack from before)
 * Updating tests to reflect that change
 * Fixed a test within the Graph section that wasn't testing the results properly for parsed dates (which is why I didn't notice a certain segment of dates not being parsed until now)
 * Test coverage remains at 100%