Custom field specified by model with the full application label problem

[fb@…]

Hi,

I run into a problem when using a Custom field and
the custom field uses a string as the reference.

I created a demo of the problem here:
​https://bitbucket.org/alien8/north

Please also see the last 2 commits. It shows the change that
breaks it.

South version is 0.7.4, Django is 1.4, Python is 2.7.1.

I tested on Mac OS X and Ubuntu 11.10.

Thanks for looking into this issue. Thanks for South! It's great!

Frank