Refactoring

* Added AccountService#getRiver - a helper method to find a river using
its unique id and to throw a NotFoundException when  the river is not found

* Added @ResponseBody to the RiversController#(add/delete)Follower methods
so that the clients don't get a 400 response

* Added an optional request parameter -"follower" to RiversController#getFollower
which when specified, returns a single follower