A user can view posts in a paginated list

* install Kaminari gem
* run bundle and the kaminari generator
* modify the generated config/initializers
* modify post model to override default kaminari config
* modify categories controller to implement pagination
* modify category#show to implement pagination
Shouldn't this be `.page params[:post]`, or even better, `.page(params[:post])`?

I'd also suggest we put the `.order` in the Post model as a `default_scope`, probably ordered by `updated_at` instead of `title`. That way we always see the newest posts first. Something like `order("updated_at DESC")`.