Add support for nodeunit.

[dossy]

This is the minimal change necessary to get nodeunit tests running
using the browserstack-runner.

Sorry, I didn't want to get all tangled up in that fancy ES6 transpiled
Adapter-pattern goodness which is JsReporters. Someone else is free to
adapt this pull request and submit it to them, but this patch satisfies
my needs with the smallest amount of code.

[Krinkle]

@dossy Hi, do you want to submit this to js-reporters instead? That way it can be easily tested and verified, and can also benefit other users of js-reporters.

[dossy]

@Krinkle I'm not actually using nodeunit any more, and the js-reporters site has this:

• Nodeunit issue (Discontinued.)

I suspect this is a dead end at this point.