Update docs to reflect inf map change from pgv to nx
Update docs to reflect inf map change from pgv to nx

This reflects recent changes in PySB, where kappa.influence_map now
returns a networkx MultiDiGraph rather than a pygraphviz AGraph.

Closes issue #320.

sorgerlab/indra,bgyori/indra,bgyori/indra,sorgerlab/indra,sorgerlab/belpy,sorgerlab/belpy,pvtodorov/indra,johnbachman/belpy,sorgerlab/indra,johnbachman/belpy,sorgerlab/belpy,pvtodorov/indra,pvtodorov/indra,johnbachman/belpy,johnbachman/indra,johnbachman/indra,pvtodorov/indra,bgyori/indra,johnbachman/indra