Fix issue when running nosetests and django_nose is not required

Hi there and thanks for this plugin, it's great to be able to have django tests running in PyUnit.

I just noticed a small side-bug when launching the `nosetests` command without `--with-djangonose` on non-django project while the adapter is installed.

Indeed, it loaded django and complained about the django settings not being defined, as the `plugin` property was called (triggering django imports) even if it was not enabled.

The fix is quite simple, and now I can keep your script installed all the time in my dev environment ...