Add Coveralls.io code coverage badge

This PR adds a [Coveralls](http://coveralls.io) code coverage badge.  

For this to work you will need to:
- Create a Coveralls account (free, trivially easy).
- Add `go-restful` to Coveralls (trivially easy).
- Update Drone.io build settings to match [this example](https://drone.io/github.com/jmcvetta/go-restful/admin), making sure to set up a `COVERALLS_TOKEN` environment variable.  Full instructions can be found at the [goveralls ](https://github.com/mattn/goveralls#droneio) project.