Add option to binarize mnist

Hi Jeff

This adds an option binarize (default to false) to Mnist, and a TableDataset:binarize() method, so as to run experiments on the binarized Mnist as done in several papers.
The new TableDataset:binarize() function mimics normalize_globally(). 

 The pipeline:binarize() available in pipe_line assumes continuous [0,1] pixels, whereas our  added TableDataset:binarize() allows any arbitrary threshold (128 is set for Mnist, following the litterature). Plus, pipeline is not documented ;-)

Thanks!