Make module `require`able as an NPM package

Even though this is not registered in the NPM registry, this change allows one to:

```sh
npm install git+https://github.com/toddmotto/atomic.git#v1.0.1 
```

assuming a new tag `v1.0.1` is published to GitHub. (Protip: `npm version patch`)

CC: https://github.com/toddmotto/atomic/issues/10