add coverage to CI; use a coverage service

[boneskull]

nyc is appropriate for Node.js coverage with all the process forking we need to do; karma-coverage probably fine for browser.

Coveralls.io is straightforward. Code Climate is probably going to have more info. Codacy is another one. There are a ton of these things. Anybody have a favorite?

[pgoldrbx]

+1 for nyc and karma-coverage (though combining the output might still have some issues due to file paths in karma)

From experience, Coveralls is nice in that you'll get per-branch/PR coverage information, versus Code Climate which currently only shows reports on the default branch. I have no experience with Codacy. It might not matter too much which you choose to start as anything is better than nothing, but being able to identify coverage changes on PRs is a pretty important OSS concern.

Code Climate does offer a fantastic environment for static analysis, though. Duplication and complexity detection are really good there. You can use these tools together, of course.

edited for typo

[coderbyheart]

Seems like a duplicate of #2351