added suggestion for nested Venn diagram using squares

Hi Konstantin,

what do you think about adding this diagram? It's for consecutive sub-selection (subsets of subsets), and also area-proportional. I used rounded squares.

call like so:

    venn_nested(sets, labels = [r'$\mathbf{%s}$ ($%d$)' % (string.ascii_uppercase[i], len(s)) for i, s in enumerate(sets)])

Cheers,
             Johannes

![probselect](https://f.cloud.github.com/assets/51015/1598039/aec9a2d6-5321-11e3-85ff-90ba6e0df56b.png)