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)

Hmm, we should probably have a quick caveat here or nearby that talks about the interaction with `polymer-ready` here:

Any code you write outside `test` functions will execute immediately (including `suite` functions). By default, WCT will wait until `polymer-ready` to run your tests, so that everything behaves as expected within them. But you may not have upgraded elements outside of them. `testImmediate` is a helper that's available for running tests before `polymer-ready`

Not sure how to word that properly; it probably won't come up in too many cases, but definitely helps to know