In Node v0.11, execute tests natively using --harmony-generators

That is, in addition to transforming `test/tests.es6.js` into `test/tests.es5.js`, we should run the tests natively using the following command:

``` js
mocha --reporter spec --harmony-generators test/tests.es6.js
```

This will ensure that we maintain fidelity with the V8 implementation of generators, and (perhaps most importantly) allow Travis CI to verify fidelity in Node v0.11 with every push.
