added ELO rankings

returns same data as players, but with new column ELO_rating, sorted in
descending order (float value with lots of decimals)

under getELORankings.php, tried to imitate output format (check please?)
+ small fix to index.php, class League, getPlayersRanking() which
output:

echo outputSuccess( array( 'players' => $players ) );

and now outputs:

echo outputSuccess( array( 'ranking' => $players ) );