Nested dicts

Hi hugh,
I've finally had some free time to do this!

When I first used your class for the project I was working on, I added the support for nested dicts in a very specialized way. I tried to do more or less the same I was doing but in a more general way and integrated into the DictDiffer class. 

I'm sure it can be improved though. For a start, the format I'm using to return the differences is not very intuitive. Again, it made sense for my case, but a different way may be better for most people. Perhaps returning a tuple of all the keys up to, and including, the one changed, instead of separating the parents.

I'll try to set aside a bit more time next week to try to improve it, and I'm sure you and others will easily improve it as well. 

Thanks again for your original code. First class.