Prints to loggers

I replaced prints statements by logging in some parts of Pylearn2 (work in progress) to resolve the issue #41

I tried to keep the form of what the author wanted to print but I also keep the technic used to print the string (some use str('string') and concatenation, others the modulo operator). The modulo operator can be a problem when I can't know the type of the value at first glance, so it might be interesting to avoid it.

There isn't a lot of logger.debug and I put it where it seems obvious, but maybe, at some place, logger.debug is better than logger.info. Let me know.