mattdrayer/api-progress-org-filter: Added new leaderboard filter

@martynjames -- added the ability to filter leaderboard results by organization -- take a look and let me know what you think.

Also just a note, this PR includes the change from the default leaderboard count of 3 -- if no count is specified it will return all users matching the filters and having a position on the leaderboard (ie, at least one completion)