fallback to six if present

While using JSONfield with Django on Windows7, I installed Jsonfield with pip. And integrated it with my app, but when I used it one my models and tried to migrate it with south. It couldnt import django.utils.six. 

Then I installed six using pip and added that in the fields.py. 

And it worked well! I m including this screenshot of the traceback of the failed import
![traceback](https://f.cloud.github.com/assets/1097043/520198/2e7b42a4-bf30-11e2-928f-9bc05c9f26aa.png)