Add stochastic Kronecker graph model to random_graphs.py

I've added two simple implementations - one O(V^2) and one "fast" O(E) - of the stochastic Kronecker graph generation model (described in ["Kronecker graphs: an approach to modeling networks"](http://arxiv.org/abs/0812.4905) by Leskovec et al.) to random_graphs.py. I also added some tests of these generators.

This is my first pull request to NetworkX - please let me know if there's anything more I should add or anything I should change.