Add implementation of GenAttack

[zimmerrol]

No description provided.

[coveralls]

Coverage increased (+0.04%) to 99.536% when pulling 58bf7e9 on zimmerrol:gen into 1a63b76 on bethgelab:master.

[codecov-io]

Codecov Report

Merging #507 into master will decrease coverage by 1.19%.
The diff coverage is 99.54%.

@@            Coverage Diff            @@
##           master     #507     +/-   ##
=========================================
- Coverage   99.49%   98.29%   -1.2%     
=========================================
  Files          45       47      +2     
  Lines        2364     2587    +223     
=========================================
+ Hits         2352     2543    +191     
- Misses         12       44     +32

Impacted Files	Coverage Δ
foolbox/attacks/__init__.py	100% <100%> (ø)	⬆️
foolbox/attacks/gen_attack_utils.py	100% <100%> (ø)
foolbox/attacks/gen_attack.py	99.04% <99.04%> (ø)
foolbox/models/tensorflow.py	37.5% <0%> (-62.5%)	⬇️
foolbox/zoo/zoo.py	66.66% <0%> (-33.34%)	⬇️
foolbox/zoo/model_loader.py	66.66% <0%> (-33.34%)	⬇️
foolbox/zoo/git_cloner.py	84.61% <0%> (-15.39%)	⬇️
foolbox/attacks/blur.py	98.38% <0%> (-1.62%)	⬇️
foolbox/zoo/weights_fetcher.py	100% <0%> (ø)	⬆️
foolbox/distances.py	100% <0%> (ø)	⬆️
... and 3 more

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[jonasrauber]

Please move all these utility functions into gen_attack.py or gen_attack_utils.py or something like that (inside the attacks folder!).