Implement String.prototype[@@iterator]

Implement `String.prototype[@@iterator]`.

http://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-string.prototype-@@iterator

Fixes issue #539
I would skip one of the hasOwnProperty checks.

Also binops before newlines

if (!isObject(o) || || !hasOwnProperty(o, iteratedString)) {