Support async request interceptors

Angular's regular interceptors support returning a promise for the config. We now need this in the dashboards so here's a pull request to add it to NgHttpFactory.

Eyes pls @harrison 
Could do this:

```
$q.when(config).then(function(config) {
  return makeHttpRequest(config, {
    responseInterceptor: responseInterceptor,
    responseErrorInterceptor: responseErrorInterceptor
  });
});
```