Stub generation doesn't respect the 'exclude' spec

Hi,

I feel that the `exclude` entries in a factory should be respected by the `stub` methods. I feel that they aren't. I've include a test which should it failing, in my view.

Perhaps this is by design? In which case, please close this, otherwise I'd be happy to try to fix the issue if desirable.

Thanks for factory_boy, I am in awe of how much easier it has made test data creation for my django app.

Cheers,
Michael