Add JSHint config

Using JSHint makes it easy for everyone to follow common code quality and formatting rules. This config is also very similar to what the [CodeClimate](https://codeclimate.com/github/remotestorage/remotestorage.js) linter will consider clean code.

Feel free to discuss these rules, although we can also do that later in new PRs. I'd rather have something as soon as possible and then improve it over time.

If you need help setting up JSHint in your editor, you should find plenty of resources via your favorite search engine.