rename Python module from 'raytracer' to 'pyrit' improve Python binding: - new objects: Container, Octree, KdTree, Shape, Pixmap, Sampler, DefaultSampler - all shapes are now subclasses of Shape - clean, remove redundant Getattr's - Raytracer render method now just wraps C++ method without doing any additional work adjust all demos for changes in Python binding, remove PIL dependency add demo_PIL.py as a example of pyrit + PIL usage render_nff.py now either loads file given as a argument or reads input from stdin otherwise fix bug in pixmap float to char conversion

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