stochastic graph
----------------
>>> import networkx as nx
>>> G=nx.DiGraph()
>>> G.add_edge(0,1)
>>> G.add_edge(0,2)
>>> S=nx.stochastic_graph(G)
>>> print nx.is_isomorphic(G,S)
True
>>> print sorted(S.edges(data=True))
[(0, 1, {'weight': 0.5}), (0, 2, {'weight': 0.5})]
