Add apm support

Add support for [apm](https://github.com/atom/apm) (atom package manager).

Adds support for version number, downloads and license through [apm's REST api](https://atom.io/docs/latest/apm-rest-api).

I didn't actually update `index.html`, just added routes in `server.js` and updated `try.html`. 
The [contribution guideline](https://github.com/badges/shields/blob/master/CONTRIBUTING.md) mentions that `index.html` is generated, but I'm not sure how.