Test using web-component-tester, change shadowRoot to Polymer's shadowRoots['ajax-form'], and update to Polymer 0.5.1

Firefox doesn't work yet for the WCT test's; the url gets passed wrong - but this could be something to do with my configuration.

This does add the requirement of WCT, which adds the requirement of Java. You can also spin up a tiny webserver, navigate to test/index.html, and run the tests in the browser manually. It's quite a bit faster than running wct.

Pull request resolves #23, closes #16, and (hopefully?) fixes #28