Add unit tests for the NSDate+Helper category.

Per [Yuan Zhu's comment](https://github.com/ResearchKit/AppCore/pull/73/#discussion_r31852445):  add unit tests for the utility methods in the [NSDate+Helper](https://github.com/ResearchKit/AppCore/blob/master/APCAppCore/APCAppCore/Library/Categories/NSDate+Helper.h) category.
