Integrated coveralls code coverage

@jdimond - As part of experiment week I wanted to start integrating our public repos with this code coverage reporter called [Coveralls](https://coveralls.io). The tool is pretty neat and works with any code coverage tool which generates `lcov.info` files. We already had Istanbul code coverage set up for GeoFire and getting it to generate `lcov.info` files was super easy. Integrating with Coveralls also took only a few lines of code but tripped me up for a little while. But I got it working now and you can see the code coverage for this check-in [here](https://coveralls.io/jobs/2581366). It shows some useful things:

* Our overall code coverage is pretty great (~95%).
* We don't have any tests for our [geohash cleanup code](https://coveralls.io/files/272524132#L82) in `geoQuery.js`.
* We should add a test for [this line](https://coveralls.io/files/272524130#L269) in `metersToLongitudeDegrees()`.
* We should add a test for [this line](https://coveralls.io/files/272524130#L379) in `geohashQuery()`.
* The rest of the untested lines are just error conditions which should not be hit in regular usage and don't really require tests. If we want, we could add ` /* istanbul ignore if */` before those checks to have Istanbul skip over them and not count them towards the overall code coverage percentage.

I think this is some useful information for us and can help us make sure our library is solid. I also added a code coverage badge to our README to show that the repo is heavily tested and should therefore lend some more legitimacy to it. Please review the changes and merge if it all looks good!