Add FGSM attack

[martinmatak]

As DeepGame seems not to be reproducible/deterministic, a new approach should be added.

One suggestion is FGSM attack: https://arxiv.org/abs/1412.6572

Cleverhans library can be leveraged for this: https://github.com/tensorflow/cleverhans/blob/master/cleverhans/attacks/fast_gradient_method.py