JSON logs for mochitests

I'm pretty happy with having the ability to output results as JSON files. This could be used to track (amount of) JS/chrome errors, warnings, all kinds of things from inside an HTML page.
Usage example: ./mach mochitest-plain --log-mach=- --log-raw=mochi.json testing/mochitest/tests/Harness_sanity/
I just noticed, it didn't log some system JS error, so it's not perfect yet, see bug 1055863.