Efficient view converter

Simplified DefaultViewConverter's view conversion methods. They now return a view, rather than a copy, whenever possible. Should help when doing trivial preprocessing (e.g. pylearn2.datasets.preprocessing.CentralWindow) of large datasets.
Can you try running the CIFAR-10 maxout YAML file and make sure it gets the same performance after this change? I doubt there is much test coverage on Travis for different image formats since most of the best convolutional features are GPU-only