add tests for ./bin/nyc.js

[bcoe]

It would be nice to have some unit tests for the nyc bin, I'm fine with these simply using spawn and asserting against stdout/stderr.

[jamestalmage]

I have found testing to stdout/stderr to be really problematic on Windows. Especially on Node 0.10 (though not exclusively). It's certainly worthwhile to include a handful of integration tests just to make sure all the glue works. Perhaps a better idea would be to run against a fixture with known coverage, and inspect the reports.

[bcoe]

@addaleax thanks \o/ see:

#224