Added underscore library.
randomChoice improvement + code refactoring to use it.

randomChoice now optionally takes an "excluded" array of elements.

randomChoice([1, 2, 3, 4], [1, 2]) will never return 1 or 2.

This helps with selecting random names from a list:

var names = ["jack", "jill", "tom"];
var name1 = randomChoice(names);
var name2 = randomChoice(names, [name1]);