Implement String.prototype[@@iterator]

Implement `String.prototype[@@iterator]`.

http://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-string.prototype-@@iterator

Fixes issue #539
It seems that the spec has been updated, and there isn't a type check anymore. It now calls ToObject, so strings turn into String objects, and it should work. Am I reading the spec correctly?