Use django-app-enabler as patching engine

[Iacopo Spalletti]

Usage of https://github.com/nephila/django-app-enabler will enable to:

• have a more robust way to patch settings.py file
• have a cleaner settings.py file (see #349 )
• allow to install third party applications during the project creation (see #381 )
• support more project templates (see #136 )

Replaces #360