Refactor cifar10 #199

In this PR, I did the following modifications to pylearn2/datasets/cifar10.py so that CIFAR10 use Preprocessor classes:
1) introduced equivalent preprocessors for the options given to the CIFAR10 constructor (center, rescale, gcn and toronto_prepro)
2) added a "preprocessors" argument to the CIFAR10 constructor
3) if the existing argument is specified, we use the new preprocessor to implement it, and issue a warning

These new preprocessor classes are defined in pylearn2/datasets/preprocessing.py (at the end): TorontoPreprocessor, CenterPreprocessor and RescalePreprocessor.

Reference: ticket #199
What is supposed to happen if `which_set == 'test'`?