Testling browser TAP reporter

We need a TAP reporter that runs in the browser to use nodeunit on [Testling](https://ci.testling.com/). I'm happy to whip one up, but I'm not sure where it belongs.

Options:
1. [karma-nodeunit](https://github.com/karma-runner/karma-nodeunit) is a separate repo. I could do the same for a TAP browser reporter. This direction feels a little funny because it's a generally useful reporter, like the [existing server TAP reporter](https://github.com/caolan/nodeunit/blob/master/lib/reporters/tap.js) and it'd be detached from nodeunit development. karma-nodeunit has the browser nodeunit build [committed](https://github.com/karma-runner/karma-nodeunit/blob/master/lib/nodeunit.js) in its source, which is a bit smelly.
2. Build it into nodeunit/dist/ with `make browser`. This direction feels funny because it's very much a second class citizen compared to the other reporters. Testling encourages test suites made of browserifiable CommonJS modules. There'd be no easy way to `npm install` and `require()` the reporter.

Thoughts?

Start by reading the existing server TAP reporter in lib/reporters/tap.js, then compare the two proposed locations: the separate karma-nodeunit repository or nodeunit/dist/ via make browser. The work is done when the placement is decided and a browser TAP reporter can be used with Testling and browserifiable CommonJS test suites.

Written by the indexing model from the issue text.