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 
hm would be really nice to update to jasmine 2.. :) also we should update angular to 1.3 as we use this everywhere.. 
when I did it for gocardless it wasn't too bad. The main one we use `toEqualData` can be written as `toEqual`