Set up coveralls for code coverage collection and reporting

[martijnthe]

[martijnthe]

@akosthekiss I'd like to add test coverage reporting to this repo. I've used https://coveralls.io/ in other JS/TS projects. It's free for open source projects. Someone that is an admin on this github org will have to log in with https://coveralls.io/ and enable it.

I can take care of the rest (adding the relevant pieces to .travis.yml, package.json, ...)

[akosthekiss]

@martijnthe I've enabled coveralls for the repo.

[martijnthe]

Thanks, will integrate it once the first feature PR has landed (#4)