Integrate Callbacks

[danielenricocahall]

Copied from:

• Integrate callback functionality into elephas (History, checkpoints, etc.) maxpumperla/elephas#131
• How to show log on every epoch ? maxpumperla/elephas#40

https://github.com/keras-team/keras/blob/master/keras/callbacks.py#L341

The fit function in Keras returns a graph that can be used to determine if the model is overfitting or not. This would be very useful from Elephas.