Rework template context extraction for compatibility with Django 1.9
Rework template context extraction for compatibility with Django 1.9

Fixes #769

tim-schilling/django-debug-toolbar,django-debug-toolbar/django-debug-toolbar,tim-schilling/django-debug-toolbar,spookylukey/django-debug-toolbar,django-debug-toolbar/django-debug-toolbar,django-debug-toolbar/django-debug-toolbar,jazzband/django-debug-toolbar,pevzi/django-debug-toolbar,pevzi/django-debug-toolbar,tim-schilling/django-debug-toolbar,pevzi/django-debug-toolbar,spookylukey/django-debug-toolbar,Endika/django-debug-toolbar,Endika/django-debug-toolbar,jazzband/django-debug-toolbar,Endika/django-debug-toolbar,spookylukey/django-debug-toolbar,barseghyanartur/django-debug-toolbar,barseghyanartur/django-debug-toolbar,jazzband/django-debug-toolbar,barseghyanartur/django-debug-toolbar