Array.prototype.shuffle to simple shuffle() function.

Extending native objects is a bad practise:
http://stackoverflow.com/questions/14034180/why-is-extending-native-objects-a-bad-practice
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Inheritance_and_the_prototype_chain#Bad_practice.3A_Extension_of_native_prototypes

Don't use it in future, please.