Method for calculating score for a given percentile value.

New methods: `score_for_percentile` and `score_for_percentile_in` (paralleling `percentile_for` and `percentile_for_in`).

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).