Add grunt task to run tests vs headless chrome

[cwmoo740]

I didn't want to make a sauce labs account to run tests against this.

This uses https://www.npmjs.com/package/mocha-headless-chrome to run mocha tests in the background vs headless chrome using puppeteer.

This also includes the commit from #50

with notes on running these tests vs sauce labs.

On a side note, I see that the build artifacts from this repository are copy+pasted into signal-desktop. I would very much like to see this protocol library become a first class citizen on npm and be a usable library for both node and web/electron, and if there's support from signal for doing this and reviewing PRs, I can make that happen.

[cwmoo740]

this is necessary because chrome has a few weird security rules:

1. crypto apis are not available on http
2. service workers are not available on http or file://

[PunkChameleon]

Had the same thought to move this off saucelabs, thanks @cwmoo740!

Willing to help on the mission to make this a module as well!

[PunkChameleon]

Cloned and ran this; all seems to work well and the only tweak I'd make is to do a wholesale switch from sauce-labs to this and replace grunt test as such with default (maybe make Sauce Labs optional, or only a release dependency?). This is much faster and lowers the barrier to entry for testing.

@cwmoo740 -- I think the blocker to upgrading this to be a module (at least is node) is the fact that node doesn't support the WebCrypto API -- looks like there's some on going work to bring it to browserland in #11 but that dates back to 2016 so I'm not sure where it sits now.