add unit test for LabeledDiGraph.remove_deadends
add unit test for LabeledDiGraph.remove_deadends

closes #92
tulip-control/tulip-control,tulip-control/tulip-control,necozay/tulip-control,necozay/tulip-control,necozay/tulip-control,necozay/tulip-control,tulip-control/tulip-control,necozay/tulip-control,tulip-control/tulip-control