Migrate index.js to ES6 & make it a plain function
Migrate index.js to ES6 & make it a plain function

- Convert to basic ES6.
- Convert `fetch` into a regular function. This is in line with the spec &
  browser implementations, where calling `fetch` as a constructor is
  explicitly not supported. (Example: Chrome returns "TypeError: fetch is not
  a constructor".) Additionally, constructors masquerading as functions is no
  longer supported in ES6.

gwicke/node-fetch