Cross validation update

I've reorganized the cross-validation stuff into a package, and added tests for more of the functionality. The main test trains a network with three pretrained hidden layers and an output layer, testing the dataset iterator, transformer dataset iterator, and stacked blocks components of that process. I would have put TransformerDatasetCV into a separate transformer_dataset_iterators.py file, but then the YAML tag for that class would be longer than 80 characters, so I left it in dataset_iterators.py.

I'm guessing this will be merged into lisa-lab/pylearn2 in the next week or so, but this will allow us to train networks with more than two hidden layers for the time being.