Added Leaderboards

#252
I know the that most achieved goals and most logged exercises aren't that exciting, but the main reason I put them in there is because if we get more things that are worthy of having a leaderboard form the functionality to implement more is already there.

I was also of thinking adding something special for the person who's rank 1, like a trophy in their user profile or something.  If you guys think that's cool or have a different idea I can add that.