http://sroucheray.org/blog/2009/11/array-sort-should-not-be-used-to-shuffle-an-array/

Array.prototype.shuffle.js

/*
 * Add a shuffle function to Array object prototype
 * Usage :
 *  var tmpArray = ["a", "b", "c", "d", "e"];
 *  tmpArray.shuffle();
 */
Array.prototype.shuffle = function (){
    var i = this.length, j, temp;
    if ( i == 0 ) return;
    while ( --i ) {
        j = Math.floor( Math.random() * ( i + 1 ) );
        temp = this[i];
        this[i] = this[j];
        this[j] = temp;
    }
};