Define page method when a AR class is defined

Commit 92052eedf047d65df71cc0021a9df9df1e2fc36e looked good at the beginning. However, that causes a bug that kaminari's page_method_name config has no effect since the pagination method will be defined when `lib/kaminari/models/active_record_model_extension.rb` is loaded.

Define that 'page' method when an AR class is actually defined so we will be able to change the name of the method properly. Fixes #481