Fixed and tested builtin graph indexes

Builtin graph indexes ({in,out,all}Neighbors{Count,Index}) where kind
of broken, I found out trying to develop a crawler inside the graph.

From now:
 * {in,out,all}NeighborsCount always contain for each node respectively its incoming, outcoming or full degree in the graph.
 * {in,out,all}NeighborsIndex always contain for each node respectivel
y an object of every incoming, outcoming or simply linked edges, group
ed by neighbors.

Also added some unit tests to keep them working.