add a package.json file, publish to npm

I've [published this component to npm](https://www.npmjs.org/package/timoxley-offset) and added `timoxley` as an owner. This module now works with browserify, however the name on npm is namespaced as `timoxley-offset`, since `offset` was already taken.

If you would merge this `package.json` file then I won't need to maintain my fork.

Cheers!