topo: add __getitem__ for Graph

Commit 65c35b65 'Remove networkx dependency' broke this line from RipL:

    nodes = [n for n in self.g[name] if self.layer(n) == layer]

To work around this, RipL code would have to be changed to something
like this:

    nodes = [n for n in self.g.data[name] if self.layer(n) == layer]

...which would use an internal variable, data.

It seems cleaner to add this one little feature from NetworkX Graph objects.

mininet/topo.py

@@ -40,6 +40,10 @@ class Graph( object ):
             for dest in self.data[ src ]:
                 yield ( src, dest )
 
+    def __getitem__( self, node ):
+        "Return link dict for the given node"
+        return self.data[node]
+
 
 class Topo(object):
     "Data center network representation for structured multi-trees."