Implement automated code coverage reporting

I'm interested in refactoring the unit tests. A code coverage report will give other reviewers confidence that those changes haven't introduced bugs in the tests themselves.

@mikesherov @caitp Does this sound good to you?

@rwaldron Pending their approval but prior to merging, I'll need you to log in to http://coveralls.io with your GitHub credentials and enable Coveralls for this repository.

> - Collect unit test coverage using the Istanbul module.
> - Update the CI scripts to report this data to the Coveralls service so
>  it can be automatically incorporated into pull requests.
> - Remove legacy code coverage file.
:-1: on hat operators please, there are some of us who still want to support Node v0.8.x!!