v0.7.1

Changes:

• Speed up network initialization via lazy parameter initialization using tensorflow's tensors instead numpy's arrays
• Global pooling layer accepts two string arguments that point to different tensorflow functions.
• Fixes for the reshape layer when used with unknown input shape
• Fixed cross entropy loss functions for spatial inputs
• Removed input blocker during the training
• Combined GradientDescent and MinibatchGradientDescent into one class GradientDescent