Associate users and articles; require login to post

Resolves #12.
I'm a little confused on this. This storage is primarily to benefit the login page (`sessions_controller`). SessionsController has `skip_before_action :store_redirect_url` and the create method redirects to the page they came from with `redirect_to redirect_url ? redirect_url : :articles`. Did you mean get rid of `store_redirect_url` and use `redirect_url` as the `before_action` method instead?