cmix v3

I have released cmix version 3. This version is over twice as fast, uses less memory, and gets better compression ratio.

Backpropagation

Today I derived backpropagation for a logistic activation function and a cross entropy loss function. This has been done before, but I couldn't find the full set of formulas for it online. I am using the variable notation from here: http://neuralnetworksanddeeplearning.com/chap2.html