`INSTALLED_APPS` in django is obviously a tuple and therefore immutable.

Why does django-userena try to append a module to it at runtime?

Reference userena/tests/profiles/test.py

```
from django import test

class ProfileTestCase(test.TestCase):
    """ A custom TestCase that loads the profile application for testing purposes """
    def _pre_setup(self):
        # Add the models to the db.
        self._original_installed_apps = list(settings.INSTALLED_APPS)
        settings.INSTALLED_APPS.append('userena.tests.profiles')
        loading.cache.loaded = False
        call_command('syncdb', interactive=False, verbosity=0)

        # Call the original method that does the fixtures etc.
        super(ProfileTestCase, self)._pre_setup()

    def _post_teardown(self):
        # Call the original method.
        super(ProfileTestCase, self)._post_teardown()
        # Restore the settings.
        settings.INSTALLED_APPS = self._original_installed_apps
        loading.cache.loaded = False

```

And obviously, when I run the unit tests with userena, I get errors such as:-

```
======================================================================
ERROR: test_can_view_profile (userena.tests.models.BaseProfileModelTest)
Test if the user can see the profile with three type of users.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./django-trunk/django/test/testcases.py", line 499, in __call__
    self._pre_setup()
  File "./_thirdparty/django-userena/userena/tests/profiles/test.py", line 11, in _pre_setup
    settings.INSTALLED_APPS.append('userena.tests.profiles')
AttributeError: 'tuple' object has no attribute 'append'

======================================================================
ERROR: test_get_full_name_or_username (userena.tests.models.BaseProfileModelTest)
Test if the full name or username are returned correcly
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./django-trunk/django/test/testcases.py", line 499, in __call__
    self._pre_setup()
  File "./_thirdparty/django-userena/userena/tests/profiles/test.py", line 11, in _pre_setup
    settings.INSTALLED_APPS.append('userena.tests.profiles')
AttributeError: 'tuple' object has no attribute 'append'

```

How do I solve this problem?