Fixes #164 - add initial guide on testing elements

reviewers: @ebidel @robdodson 

Preview: https://github.com/Polymer/docs/blob/4b8e748dad6007bc859ae057ba6f20610fa25a0a/articles/unit-testing-elements.md

I plan on following this up later in Q4 with a more detailed end-to-end walkthrough, but this will at least give us something until then. Go go testing!

![giphy 30](https://cloud.githubusercontent.com/assets/110953/4334791/f350da40-3ff3-11e4-823e-920b60a79c37.gif)

Inclination is to distill the reasoning text into something more terse. I
think at present it may be TMI for a guide but we can summarize.

On Thu, 23 Oct 2014 20:22 Ian MacLeod <notifications@github.com> wrote:

> In articles/unit-testing-elements.md:
>
> > +## web-component-tester
> > +
> > +The Polymer team created [web-component-tester](https://github.com/Polymer/web-component-tester) to make testing a breeze. It uses some familiar tools for unit testing elements - namely, [Mocha](http://visionmedia.github.io/mocha/) and [Chai](http://chaijs.com/). Using Mocha as our testing framework allows us to write tests that support development and integration without the need for workarounds.
> > +
> > +Mocha (as you probably know) was designed to be flexible and needs a few extra pieces in order to make it complete. Perhaps the most important extra piece we use is a JavaScript assertion library called Chai.
> > +
> > +Chai supports a lot of the assertion styles you might want to use (expect, assert, should). The real difference in these styles is their readability and you’ll likely pick a style that fits in with your or your teams own preferences.
> > +
> > +Out of the box, web-component-tester includes:
> > +
> > +* [Mocha][mocha] as a test framework.
> > +* [Chai][chai] assertions.
> > +* [Async][async] to keep your sanity.
> > +* [Lodash][lodash] to repeat fewer things.
> > +
> > +Additionally, WCT provides integration with `selenium`, so that you can easily run your test suites across multiple browsers.
>
> Hmm, let's downplay the selenium point (it's an implementation detail of
> the browser launching) - it really becomes more useful once we expose
> selenium commands to the tests
>
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> Reply to this email directly or view it on GitHub
> <https://github.com/Polymer/docs/pull/650/files#r19301091>.
>