Add Python binding to another overload of segNet::Mask

[hanzhi713]

Addresses issue #473

• Add binding to another overload of segNet::Mask
• Use integer division in segnet-camera.py

Usage of the new python binding:

# classes is an uint8 array
classes = jetson.utils.cudaAllocMapped(width * height)
net.MaskClass(classes, width, height)
# cudaToNumpy assumes float32. Need to dividie output length by 4.
mask = jetson.utils.cudaToNumpy(classes, 1, width * height // 4, 1)[:, 0, 0]
# convert resulting float32 view to uint8 view
mask_np = mask.view('uint8').reshape(width, height)