Implemented the new leaderboard endpoint (there only is a weekly leaderboard), since the leaders endpoint (with range) didn't return the expected data. It did return friends only without ranking or other leaderboard data.
Renamed the weekly_leaderboard method to leaderboard, since there isn't another type of leaderboard.
Merge pull request #19 from kevintuhumury/master

Replaced the leaders endpoint with the actual leaderboard endpoint.