Change User model to_param
`to_param` now returns username attribute instead of id.
Now all default routing, i.e. from `user_path` routes on `username`
instead of `id`, which is prettier and what reddit does. One gotcha, though:
`resources :users` uses routes like 'users/:id', so the username is
accessed through `params[:id]` since I can't figure out how to override
that behavior.