Migrating to the new web-component-tester client setup.

Depends on https://github.com/Polymer/web-component-tester/pull/8

Pushing this up for feedback on the testing style & structure. Largest changes:

* `tests/index.html` instead of `tests/runner.html` (https://github.com/PolymerLabs/seed-element/issues/22).
* Each test file only needs to include `web-component-tester/client.js`.
* `index.html` calls `WCT.runSuites(['foo.html', 'bar.js', ...])` to load and run the sub suites.
* Each suite can use Mocha's `suite`/`test`/etc - in `.js` or `.html` tests.

**Don't merge yet.**