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, I'm a little worried that people might pick this up as a pattern (writing tests for elements defined in another project). Maybe this could be framed as adding a `core-selector` to `seed-element` as a feature, and then testing that?