Added django-mptt in categories

Django-MPTT uses MPTT to store trees, which is much more efficient than plain foreign keys. It is therefor better and it also manages category level by itself so there is no need to maintain code for that. 
There is only one problem. Root level is 0 but in LFS it used to be 1. I tried to trace all places that were changing level. Those could cause problems. There might be a problem with templates as well.
Also be careful with south. New version requires field definitions but I wasn't able to google what should be there.