Implement unauthenticated request handling via $http interceptor.

* Add interceptor to "$http" service which intercept unauthenticated
  responses (401) and invoke the "$auth" service's "unauthenticated"
  method.  All requests pending at the time will have an "ignored"
  property added to their configs that means that the same intercept
  logic will not be executed if they return a 401.

* Add logOut method to "$rootScope" in the app's "run" block.

* Change CSRF response behavior to nullify session (default) instead
  of raising exception.