Update the SDK to export an actual ES2015 module

**PRs welcome!**

Currently the library is [exported in such a way](https://github.com/firebase/geofire-js/blob/7ef8c5c2129033fc641ad78fa44ddd12a9528230/build/footer#L4-L7) that you can only use `require('geofire')` within Node.js. This means you cannot get `require('geofire')` or `import * as GeoFire from 'geofire'` to work with ES2015-compatible browsers or browser bundlers like webpack and Browserify. We should export an actual ES2015 module to support those modern use-cases.

Start by reading the linked build/footer entry, then trace how the library package is assembled and exported. Verify the resulting package supports both require('geofire') and import * as GeoFire from 'geofire' in ES2015-compatible browsers and bundlers such as webpack and Browserify.

Written by the indexing model from the issue text.