Included another activation function: ReLU
Included another activation function: ReLU

This commit adds a smoothed approximation to the biologically plausible
“rectifier activation function”.

skymanaditya1/python-neural-network,ZenDevelopmentSystems/python-neural-network,jorgenkg/python-neural-network,DucQuang1/python-neural-network,sudhanshuptl/python-neural-network,vibhatha/python-neural-network