don't store a reference to the User class

The issue: `Kracken::Config` was storing a reference to the `User`
class. In development, if Rails detects that an app's source has
changed, it would reload the source (including the User class).  This
would leave `Kracken` with a reference to an old version of the `User`
class.

The (partial) fix: don't store a reference. When we need the class, get
it via `::User` and don't store it in an instance variable. This wont
solve the case where the app sets a custom `User` class, but none of our
apps do this currently.

Author: syoder

lib/kracken/config.rb

@@ -3,12 +3,16 @@ class Config
     attr_accessor :app_id, :app_secret
     attr_writer :provider_url, :user_class
 
+    def initialize
+      @user_class = nil
+    end
+
     def provider_url
       @provider_url ||= PROVIDER_URL
     end
 
     def user_class
-      @user_class ||= ::User
+      @user_class || ::User
     end
   end
 end