Version 2.0.0 with batch support, model and defense zoo, threshold support and more

• batch support: check out the new example in the README
• model and defense zoo: https://foolbox.readthedocs.io/en/latest/user/zoo.html
• attacks take an optional threshold argument to stop attacks once that threshold is reached

foolbox.attacks now refers to the attacks with batch support. The old attacks can still be accessed under foolbox.v1.attacks. Batch support has been added to almost all attacks and new attacks will only be implemented with batch support. If you need batch support for an old attack that has not yet been adapted, please open an issue.