Fix FastFixtureTestCase to support testcases without fixtures.

[teeberg]

Since Django 1.7, using the FastFixtureTestCase without specifying a list of fixtures to use raises this exception:

Traceback (most recent call last):
  File "nose/suite.py", line 209, in run
    self.setUp()
  File "nose/suite.py", line 292, in setUp
    self.setupContext(ancestor)
  File "nose/suite.py", line 315, in setupContext
    try_run(context, names)
  File "nose/util.py", line 471, in try_run
    return func()
  File "django_nose/testcases.py", line 49, in setUpClass
    cls._fixture_setup()
  File "django_nose/testcases.py", line 72, in _fixture_setup
    'database': db})
TypeError: call_command() argument after * must be a sequence, not NoneType

triggered here:

call_command('loaddata', *cls.fixtures, **{'verbosity': 0,
                                           'commit': False,
                                           'database': db})

Since Django 1.7 sets fixtures to None by default, django-nose should support this possibilty too. :-)

[jwhitlock]

I also think the way forward is to re-sync w/ the current Django code. I'd like tests for FastFixtureTestCase (issue #220) before I start merging fixes.