Proposal: use Nightwatch.js + Selenium for automated browser testing

As the mobile app becomes more complex, we should implement automated browser testing.

It looks like using [Nightwatch.js](http://nightwatchjs.org/) with [Selenium](http://docs.seleniumhq.org/) is fairly straightforward. With Selenium, we can also outsource our browser testing to [Browserstack](https://www.browserstack.com/automate).