Use the Django>=1.7 app registry

[jsocol]

• Drops support for anything < 1.7.
• Use django.apps.apps.get_app_configs() for template loaders and helper discovery.
• Delays environment creation as long as possible, and caches it, should fix #50.
• Removes direct access to jingo.env, use jingo.get_env() instead.
• Removes env dependency from Registry().
• Moves Template() higher in the module and sets Environment().template_class directly.
• Adds django.contrib.admin.apps.SimpleAdminConfig to test settings to ensure support for AppConfig classes, fixes #68.