Fix svhn_preprocessing

For the issue: https://github.com/lisa-lab/pylearn2/issues/452

It was explained the the README file that data should be manually copied to SVHN_LOCAL_PATH. And then it modifies data there. But now the svhn_preprocessing.py will copy the data if they do not exist at SVHN_LOCAL_PATH.
The float64 bug + some other bugs introduced with recent changes are now fixed.