Add elo ranking for team and players

Based on that : http://www.bonziniusa.com/foosball/tournament/TournamentRankingSystem.html
Exemple on players :
![127_0_0_1_8080___stats_players](https://f.cloud.github.com/assets/1484406/2291927/b1d5b830-a04b-11e3-9418-c9be9476ba8b.png)

Exemple on teams :
![127_0_0_1_8080___stats_teams](https://f.cloud.github.com/assets/1484406/2291929/d99abafa-a04b-11e3-99c9-d92d51e245da.png)