Fix for error 'Settings' object has no attribute 'DATABASE_ENGINE' (#208)

[pzinovkin]

Error on fresh installed master while trying to view query explain:

lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "lib/python2.7/site-packages/debug_toolbar/views.py" in sql_explain
  85.         if settings.DATABASE_ENGINE == "sqlite3":
File "lib/python2.7/site-packages/django/utils/functional.py" in inner
  173.         return func(self._wrapped, *args)

Exception Type: AttributeError at /__debug__/sql_explain/
Exception Value: 'Settings' object has no attribute 'DATABASE_ENGINE'