Add browser testing to CI

[ndhoule]

This probably involves some test refactoring work, but moving to Browserify means it's easy to add a Node shim without really thinking about it, and some of Browserify's shims rely on an ES5 environment. Would be easy to break IE6 without noticing.

[boneskull]

I'm more than ready to drop IE6 support

[boneskull]

@mochajs/mocha We need automated tests for Mocha which run in the browser.

Anyone have any opinions on the best way to get this done?

When I need to run client-side tests, I reach for Karma. If I need to debug something, I tell it to use a real browser, but otherwise I run headlessly with PhantomJS.

If there are any services out there that would automate the tests against real browsers for free, I'd like to know about them.

[ndhoule]

I generally go for Sauce Labs to run cross-browser tests. Free for OSS projects.

Karma's fine for most cases but its Sauce runner has a long-standing concurrency issue that has made it difficult for me to run tests against multiple browsers in the past. Might be able to get around this by only running one browser at a time through Karma. I prefer Zuul these days; some annoying edges but it doesn't involve the same level of configuration Karma does and works OOTB with Browserify.

[dasilvacontin]

Dupe of #2079.