Adding ability to look up a list of products a user has purchased

#1874 - Uses the `edd_get_users_purchases()` to get a list of purchases, and then returns an array of post objects for the unique products the given user has purchased.

Also includes the unit tests (first run at ever writing one for EDD so if I didn't follow your exact methods, let me know and I'll rework them).

This is my first pass at this idea, if you see changes let me know. Comments are a bit verbose, but just to help with code review. They can be removed if you think they need to be.