First successful PHPLeague's "Plates" template display

* We can display a "Hello world" through Slim and a Plugin action with only 13 included files
* Some refactoring on Plugins Packing Behaviours (including a rename of them)
* Plugins can now subscribe new Packers. Oh, this is so meta! :-)
* added a TemplatesPacker and a View Service in the "core" Plugin
* added a "Plates" PHP Pack