Add forEach method to Gremlin and TraversalWrapper.

@mhfrantz 

[Delivers #83517854]
Since promisedProcess is wrapped via Q.promised, which allows parameters to be promises, I think you can do this as a shortcut:

```js
return promisedProcess(javaIterator.next())
  .then(function () {
    return _eachIterator(javaIterator, promisedProcess);
  });
```