Infra: Introduce ESLint

[EthanLin-TWer]

• http://eslint.org/docs/user-guide/getting-started
• https://docs.travis-ci.com/user/languages/javascript-with-nodejs/
• existing eslint styleguide: https://github.com/linesh-simplicity/frontend-nanodegree-arcade-game/blob/master/.eslintrc.json

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• npm install -g eslint
• yad eslint
• run eslint --init or copy existing eslint styleguide
• add eslint js/ test/ to npm scripts
• add npm run lint to .travis.yml but disable it for the moment
• enable eslint in IDE