_add_edge and MultiDiGraph parameters

Hello,

I may be missing something here - but there seems to be an issue with https://github.com/davebshow/ipython-gremlin/blob/master/gremlin/resultset.py#L169 and the current version of networkx. I get the following error message:

 /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/ipython_gremlin-1.0.0-py3.5.egg/gremlin/resultset.py in _add_vertex(self, v)
     167 
     168     def _add_vertex(self, v):
 --> 169         self._graph.add_node(v.id, {'label': v.label})
     170 
     171     def _add_edge(self, e):
 
 TypeError: add_node() takes 2 positional arguments but 3 were given

Changing it to the following seems to resolve the error:

self._graph.add_node(v.id, label=v.label)

https://networkx.github.io/documentation/latest/reference/classes/generated/networkx.MultiDiGraph.add_node.html