Added a check for date before calling relativeDateStringFromDate.

Integrated notifications so if datasource has started updating on main page to signal when its finished if user went to categories screen.
Want to remove observer right after we get a notification

The block parameter is nil, the main queue will be used by default.

suggestions from @bskinner
Create strong reference.
Fix for if view controller is popped before notification is called we won't remove the observer.

help from @bskinner
Merge pull request #154 from MIT-Mobile/3.6/ipad-news-category-notifications

Integrated notifications and added data check before calling relativeDateStringFromDate