Add forEach method
Update/pin dev dependencies.
Use the standard `substring` instead of `substr`.

Only the former is officially part of ECMAScript.
Make `forEach` behave like I'd expect.

Just like `Map` and `Set` from the ES6 draft, it calls back with `(value, key, dict)`. Also, it no longer does three loops through the properties (viz. `Object.keys`, `map`, and `forEach`).

Closes #5.