Added a basic error handler on our Application, more precise PHP packing control

* We now have a "error()" method for our Application. It tries to mimic Silex's error handler, on top of Slim. Should need more work...
* Added a "[packing]" app config section. We can now have a more precise PHP packing control.
* Added 2 constants on our ApplicationInterface for "later" and "early" events definition
First successful Ajax navigation for this new architecture. Yeah!

* added our good ol' "ajax-navigation" Plugin
* added JS config data management
* added our "core" & "utils" Plugins JS assets
* our temporary debug info (script duration, included files...) are not displayed when a Ajax request is made