ENH Added wrapper for overfeat binary, removed passthrough layers and added tests

TODO:

- tests to verify layers one by one
I guess in this case I could say `net_weight_file = os.environ.get("OVERFEAT_NET_WEIGHT_FILE", get_net_weight_dir(net_weight_dir, overfeat_dir))`, I guess this does lazy evaluation, but I don't know if it's more readable. How would you use an OR?