Add in tests for instance based navigation controller functionality

You can now pass `nav_controller: MyNavigationController` to PM::Screen.new.

~~~ ruby
class MyNavigationController < PM::NavigationController; end

screen = PM::Screen.new nav_bar: true, nav_controller: MyNavigationController
~~~

I also reordered a navigation controller spec in screen spec for a bit more intuitive coverage of screen instantiation with navigation controllers. 

Let me know how this looks and then I can go about updating the README/wiki per project standards.