add basic linting to request library

- Add eslint as devDependency for linting
- Add `.eslintrc` file to define our custom rules
- Define `lint` script in package.json to lint the library. You can run this manually via `$ npm run lint`

List of custom rules taken from #1106, based on proposal and comments. 
Changes and bikeshedding still allowed and even encouraged (Check http://eslint.org/docs/rules/ for any rules you'd like to see added as well that aren't enabled by default)

/cc @mikeal @nylen & everyone else