Added resize parameters for dataset image reader, and the ability to loa...

...d data from alternate directories

cc @eickenberg 

Added the ability to specify arbitrary directories for the load_sample_images and load_sample_image functions to enable loading "other data" easily. 
How useful is this generalization? I can see it being pretty awesome if instead of a `read_dir` one could specify a `read_glob`. That way we wouldn't be stuck with JPG either and could load anything that PIL can load, e.g. PNG images. I think it's just a matter of importing `glob` and using it instead of `os.listdir`