Use app framework for initializing in Django 1.7, refs #64

django-constance breaks in Django 1.7 with an `ImproperlyConfigured` exception. This is mainly due to checking for model installation too early. This patch adds an `AppConfig` object which instantiates the `Config` class only once Django's app installation is done.

In my tests it ran transparently in Django 1.6 and 1.7.