Directed Gnp model (fast version) has wrong start index

For the directed graph version of the Gnp model the starting target has to be at v=0 not v=1 or no out-edges will ever be generated from node 0.
Fix failing test in planted_partition_graph

Update size to 218
Merge remote-tracking branch 'upstream/master'
added a test case for the bug and refactored the tests for the G(n, p) random graphs
removed unnecessary test code
Merge branch 'fast-gnp' of github.com:hagberg/networkx into fast-gnp
Merge pull request #1070 from hagberg/fast-gnp

Directed Gnp model (fast version) has wrong start index