changed get_wsgi_handler to application.

The expectation is that the user will define the callable application
object in their wsgi module. Previously we have been using instances of
the WSGIHandler class in django.core.handlers.wsgi.

Django recommends using get_wsgi_application() from django.core.wsgi;
which is just a function that returns WSGIHandler.

For more information on the application object, see PEP 333
http://www.python.org/dev/peps/pep-0333/