Add GET /users/:user/activity and DELETE /users/:user/activity/request/:...

...request routes to create the activity feed API and write specs, and update README accordingly
It's not the only way, but it seems as good as any other alternative: http://stackoverflow.com/questions/11637353/comparing-mongoose-id-and-strings, the problem being that they're stored as objects and are compared by reference ids if you don't turn them into strings.