Implement String.prototype[@@iterator]

Implement `String.prototype[@@iterator]`.

http://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-string.prototype-@@iterator

Fixes issue #539
This should probably use maybeAddFunction as well. I I know V8 is close to shipping `String.prototype[Symbol.iterator]` and I don't think we should override existing methods.