Switch to using web-component-tester

@nevir To get a little experience using web-component-tester, I revived your PR to add support to seed-element for it and made the following changes:

* Bumped and tested with WCT 0.6.2 (the previous PR was several versions behind)
* Cleaned up some extra spacing we no longer needed
* Corrected the ignore for node_modules (it was previously using /node_modules, whilst it ignores fine in the shorter form)

Something to consider:

The tests directory is currently ignored in bower.json. Should we do the same for the gulpfile.js & package.json files or was keeping them a part of the package intentional?
I often struggle with best practices for `Polymer/polymer` dependency versioning.

`^0.4.1` is [more restrictive](https://github.com/npm/node-semver#caret-ranges-123-025-004) than `>=0.4.1 <1.0.0`, in that `0.5.0` won't be allowed, right?Are there breaking changes planned for `0.5.0`?