Back button support

Another small change to support a back button navigation item.

Use it as normal, passing `:back` as the side to `set_nav_bar_button`. For example:

```ruby
class BuddyAddScreen < PM::Screen

  title "Add a Buddy"

  def on_init    
    set_nav_bar_button :back, title: 'Back', style: :plain, action: :back
  end
end
```