Removing the app constant and replacing it with Rails.application

Instead of thinking of a Rails application as a class, we should instead try to start thinking of them as instances of a class. This class level abstraction makes sense when the Rails application is a singleton, but in the future if there are multiple applications per class, then it does not make sense to configure the entire class, but instead makes sense to configure a particular application instance.

In this vein, I've replaced the ``app_const`` with the canonical ``Rails.application``. The behavior should be the same, but this is a step in the direction of multiple applications.

I've also allowed you to call ``configure`` on the instance instead of the class. Now, when the ``configure`` method is called on a subclass of ``Rails::Railtie``, the configure block is sent to the ``instance``.