[wip] pretty printing

I made this pretty-printing version that works in the IPython Notebook:
http://nbviewer.ipython.org/github/bollwyvl/nesting/blob/repr-html/README.ipynb

It probably doesn't work with rollup, etc. but it at least sort of works for the basic, multi-key case.

I don't know if you'd want to bring it in: no hard feelings if not! Another approach would be to make it a separate package that monkeypatches your values. Either way, I just wanted to get it out there!