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.
I've removed the warnings, but I'm thinking of leaving single_tensor in for now. Given that there's no stereo Space class, I thought that it might be useful for NORB to be able to return separate monocular design matrices, rather than a single stereo one. (This is speculation; I don't know myself, because for my own experiments I preprocess NORB into a monocular dataset using a custom script). If you don't think this will true, then I can remove single_tensor.