Added a fourth layer to the network.

It is hard coded and hacked in. Want to be see that it will work
before spending time making it pretty. It's a spike they say.

No errors, but tests fail.
All tests pass.

Notice the 4 layer network learns MUCH slower. Need smaller
learning rates and more iterations. Some refactoring would be
good as well, very repetitive code.
Changed order or operations on back propagation.

Now always change a deeper level before calculating deltas
on a shallower level. Still lots of repeated code.
Increased number of iterations used in test.

I'm continously surprised by how long it takes to train 4 layers.
Refactored back propagation.

A step in the right direction at least. Delta calculation and
weight updating algorithms factored out.
Edited to reflect the additional layer.