Invoke django-admin.py over Python API rather than as external executable - fixes #625

Relying on the presence of an executable named `django-admin.py` can be brittle for platform-specific reasons as detailed in https://docs.djangoproject.com/en/1.7/faq/troubleshooting/#troubleshooting-django-admin-py , leading to issues such as #625. This patch sidesteps the whole thing by invoking the django-admin command via the Python API that Django provides for exactly this purpose...