Add functionality to Field Dataset loader

[belsten]

Currently sparsecoding.datasets.FieldDataset only loads the whitened images (IMAGES.mat) from Bruno's site. It would be nice to have an option to load the unwhitened images as well (IMAGES_RAW.mat).

Additionally, quilt is automatically used to extract patches. However, I'd guess that people would most often want to extract random patches. It would be nice to add a feature allowing the user to select which method they want to use to extract patches (quilt vs. sample_random_patches).