Method for calculating score for a given percentile value.

In other words, we can now ask, "What's the 90th (or whatever) percentile
score in leaderboard XYZ?" Potentially useful for gleaning the overall
distribution of scores in a leaderboard (e.g. showing deciles).
Merge pull request #15 from agoragames/score-for-percentile

Method for calculating score for a given percentile value.