Added an option to dump the config reference

As promised before my vacations, here is the config reference dumping.

The ``ReferenceDumper`` class is deprecated as of Symfony 2.4 (scheduled for removal in 3.0) becuse of the introduction of the XML dumping support in 2.4 (the deprecated class extends the new class). However, I'm using it here to keep the support of Symfony 2.3 without doing a class_exists check for the new class.

A next step for this will be to provide some description of the config for nodes where it makes sense (see the custom extension in the feature file)