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)

No longer necessary - `expect` is also [made available by default](https://github.com/Polymer/web-component-tester/blob/master/browser/environment/chai.js#L12). However, if they want chai's should style assertions, they will need a setup step.