Add CW attack

[martinmatak]

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

One suggestion is CW attack introduced in: https://arxiv.org/abs/1608.04644

Cleverhans library can be leveraged for this: https://github.com/tensorflow/cleverhans/blob/master/cleverhans/attacks/carlini_wagner_l2.py