Define page method when a AR class is defined

If we define the page method within AS::Concern's ClassMethods module, it will be defiend when lib/kaminari/models/active_record_model_extension.rb is loaded. Thus, kaminari's page_method_name config has no effect since the config file(e.g. config/initializers/kaminari.rb) is usually loaded after all the gems are loaded.

Define the method when the AR class is actually defined so we will get the config working again. Fixes #481