Give frame work to add additional activation functions

[schrummy14]

This, should, allow the use of additional activation functions

[shiffman]

This is great! Do you think it makes sense to refactor sigmoid() and any future functions to be static() methods of NeuralNetwork or namespaced in some way?

[schrummy14]

For a "toy" I don't see the harm in people being able to call the function from outside the nn but that is something that definitely could be done to make sure the functions work the way their suppose to.

[shiffman]

Great, merging in for now!