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!

typo "centriv" (also drop the comment about iframes ...guiltily we do use 'em under the covers - but no gnarly workarounds!)