Packers small bugfix

* Plugins Packers don't throw Exceptions when they don't find data for their "plugin config" key (except for the GeneralPacker, which requires the "@general" key to be defined)
Some general bugfixes & improvements

* Added "after" & "before" methods to our Application
* No more Composer autoloading for PHP classes included in the Plugins Packs
* Added some missing classes in "app boot classes" & "slim" Packs profiles
* Added the "app/plugins/" folder to the gulp watch task for "live repacking"
Added automatic Pack Profiles management

* our PHP Pack Profiles are ran after each request if the  "debug/packing.always_repack_profiles" flag of the app config is set to 'true'
Some fine-tuning on logs & automatic repacking