Add an example of a route with nested path parameters - /pets/{petId}/friends/{friendId}

I spent some time today trying to find out how to annotate a more complicated route (a route with two path parameters like `/pets/{petId}/friends/{friendId}`). 

After I found out how to do it I decided to integrate it into this useful sample application.

I also fixed some stuff related with the project
* Two plugin dependencies were missing in plugins.sbt. This was causing some errors while loading the project in sbt.
* The baseUrl was set to "localhost:8080" and I changed to "/" so that it is always set to the host and port that you use to access swagger-ui.