Implement the `@@iterator` method on generators.

Per the spec [1], all this method does is return the generator itself. This is necessary to make generators from regenerator work properly with the spread element, which uses the `@@iterator` property on objects.

[1]: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-generator.prototype-@@iterator
Merge pull request #120 from eventualbuddha/add-generator-at-at-iterator

Implement the `@@iterator` method on generators.