Add node_moved signal to be sent after node successfully moved

The signal behaves similar to the normal Django pre-/post_save signals.

The reason for this feature is that I'm writing a system which has inherited permissions. And when I move nodes around I need to re-check if permissions on descendants needs to be reset. The other solutions I tried wasn't working satisfactory and added quite a bit complexity. So by adding this to django-mptt I made the entire solution much cleaner.

I hope you will also find this useful or at least harmless enough so I don't have to maintain a fork. :)