Introduce ESLint

[backflip]

• Add ESLint, remove JSHint and JSCS. Reasons:
  • One linter is more fun than two (specifically if the two are overlapping) and JSCS is in the process of being included in ESLint anyway: http://eslint.org/blog/2016/04/welcoming-jscs-to-eslint
  • .jsx proved to be difficult to handle with JSHint
• Generate an .eslintrc from previous .jshintrc and .jscs files (using https://www.npmjs.com/package/polyjuice). Minor adaptations where needed (see specific commit).
• Adapt code base to new ESLint config (where the config could not be adapted):
  • Having one single var declaration but separating require is not (yet) possible with ESLint (might be fixed in the future https://github.com/eslint/eslint/issues/6175). That's why I have combined these declarations in the .data.js files. An alternative would be to disable the one-var rule.
  • Some very specific details did start to throw errors. I have "solved" this using // eslint-disable-line

[orioltf]

I did use "extends": "eslint:recommended" (http://eslint.org/docs/rules/) for another project. How would you see that? It sets some rules that we can then overwrite with ours. This produces quite a shorter .eslintrc file.

I would also suggest to sort the rules alphabetically. This is as well how they are presented in their documentation and adds some easiness and expectation when reading the configuration.

[backflip]

@orioltf, alphabetical sorting added.

Do you want to go through the rules and compare them with eslint:recommended? Or should we do this in a second step (ideally after having tested the auto-generated config in some real projects)? I kinda like the current, explicit config since it might help switching from JSHint/JSCS to ESLint by allowing you to compare the current with the previous configs.