Move loading MNIST images into common directory.  Some more cleanup.

Since the neural network stuff uses the mnist images as well, move the
code to load and preprocess it into common.mnist.

Be better about finding modules/files and using relative paths across the board.
Merge remote-tracking branch 'origin/master'

Conflicts:
	ex1/ex1c_softmax.py
Remove solutions.

Put solutions in a personal branch.