Support Game Center's sort order for scores.

... report a score to Game Center when the new score is lower than the old score if the game’s sort order is Low to High.

![screen shot 2013-06-29 at 12 23 01 am](https://f.cloud.github.com/assets/604300/725512/a0aa8204-e073-11e2-86b9-1595dcf345ac.png)