Coding style and other improvements

[fox91]

I would like to do these improvements.

Coding style:

• [ ] code indentation: 2 space
• [ ] linting utility: switch to ESLint
• [ ] start using google/eslint-config-google

Build system:

• [ ] switch to Gulp (or yarn/npm) or at least upgrade Grunt and plugins to latest version

@bielfrontera and other contributors: let me know what do you think.