Added promise support to node client

Added promise support to the node client, without sacrificing backwards compatibility.

```coffeescript
ejs.Request()
  .indices("tweets")
  .types("tweet")
  .query(ejs.TermQuery("user", "kimchi"))
  .doSearch()
  .then (results) ->
      console.log(results)
```

Note that you cannot use both callbacks and deferreds; If a callback is passed in, it will be used instead of the deferred's callback.