Change the shuffle to the Fisher-Yates shuffle

The current "Shuffling Array Elements" page recommends a naive algorithm, which produces a slow and biased shuffle. Included is a highly idiomatic refactorization of the Fisher-Yates shuffle (aka the Knuth Shuffle), a less-idiomatic-but-better refactorization, and a version of the algorithm that adds to Array.prototype, for those that program in that style.