Add ability for services to return a promise instead of a callback

If a service returns a [Promises/A+ Promise](http://promises-aplus.github.io/promises-spec/) and the callback isn't called, the promise should be used to resolve the request.

``` js
var todoService = {
  todos: [],

  // Return all todos from this service
  find: function(params) {
    return Q(this.todos);
  }
}
```
