Fix up our manage.py for Django 1.3 deprecations.

The setup_environ() and execute_manager() methods were deprecated, in favor of
the new execute_from_command_line() and setting the DJANGO_SETTINGS_MODULE
variable. These were deprecated in Django 1.3 and removed in 1.5. We inherited
these by virtue of the fact that we modified the old manage.py--this updates
our code to look more like the new manage.py.

Testing done:
* Ran unit tests
* Used the devserver a bit

Reviewed at https://reviews.reviewboard.org/r/4589/