new NORB dataset wrapper, with image browser script

The current NORB dataset wrapper requires the dataset to be stored in a binary format that is not publicly available.

This pull request contains an alternative, pylearn2/datasets/norb.py, whose SmallNORB class can read the NORB dataset in its original format, downloadable from http://www.cs.nyu.edu/~ylclab/data/norb-v1.0-small/.

I've included an interactive image browser (scripts/datasets/browse_small_norb.py) that allows you to select images to view using their multidimensional label (category, instance, viewpoint elevation, viewpoint azimuth, lighting).

Screenshot:
![browse_small_norb](https://f.cloud.github.com/assets/5941426/1543828/d17469f2-4d5d-11e3-945f-b5cbec15d63a.png)