Added basic support for Django apps registry

There is a new feature added to Django in version 1.7 called *Applications*. It's a registry that keeps track of all applications from `INSTALLED_APPS` and their properties like name, model module, path etc. (please refer to documentation: https://docs.djangoproject.com/en/1.7/ref/applications/).

It might be beneficial for Django CMS to integrate this feature and it the future we can use it to obtain some info contained it the registry in cms code (see how it's done in `django.contrib.admin`). For now on I only propose to support `cms` and `menus` applications as a valid Django apps. More support would come later when Django 1.7 would become minimal required version for cms and menus.

Older versions of Django (< 1.7) are ignoring this and noting breaks.