Added JSCS support - Closes #65

Overview of what I did:
- added initial `package.json`
- added JSCS (use it with `$ npm run test`)
- added first rule to JSCS for quotation marks
- normalized all quotation marks based on JSCS

Where to go from here:
I didn't integrate Husky for pre-commit hooks yet. While I use Husky myself in other projects, these projects are closed and have no external contributors. So they are different from this one. A contributor would _need_ to call `$ npm install` at least once after cloning this project so Husky can set up the pre-commit hooks. If not one could technically create a pull request which isn't validated by JSCS.
You maybe want to integrate Travis which could run `$ npm run test` on every pull request. It would show you if the pull request passes all tests.

Also the `.jscsrc` needs some more configuration to be useful. I just added one rule for now: quotation marks. It looks like you used `"` most of the time so I converted every `'` to a `"`. (So most OSS projects seem to prefer `'`.)
My advice: If you really know that you want a specific style for something, then add it now to the `.jscsrc`. If not, just fine-tune the `.jscsrc` after every pull request that doesn't matches your style.

The `package.json` can now be used for more tooling. The `node_modules/` will be ignored by Git.
It's valid ES6, yes. For such simple functions, it just feels nicer.