Added new version of NORB that supports both small and 'big' NORB datasets, uses memmap caching.

The new NORB class is in datasets/new_norb.py. The new browser is in scripts/datasets/browse_norb.py. Added deprecation warnings to the old datasets/norb.py and scripts/datasets/browse_small_norb.py. 

New browser also supports swapping the left and right images so that you can view them in 3D by crossing your eyes. (use the --stereo_viewer flag). Ian requested this feature last year when I first committed norb.py.
Python3 will use floating-point division for `label / 2`, so you should probably use `//`, or `label % 2 != 0`.