Method for calculating score for a given percentile value.

More-or-less direct port of this PR from the Ruby version: https://github.com/agoragames/leaderboard/pull/43

So now we can 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).