A rewrite of http://neuralnetworksanddeeplearning.com/chap1.html#implementing_our_network_to_classify_digits
MNIST data http://yann.lecun.com/exdb/mnist/
IDX file loading http://g.sweyla.com/blog/2012/mnist-numpy/
Load shaped data from scratch using load_mnist() ( PY3 compatible ) or from a pickle file ( PY2 only ) using load_data_wrapper()