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?
Since all elements are versioned with >=0.4.1 <1.0.0, wouldn't using a restrictive version like ^0.4.1 lead to manual version resolutions?

If I decide my seed-element should depend on paper-tabs^0.4.1, then bower installs paper-tabs 0.4.1, which itself depends on core-selector >=0.4.1 < 1.0.0 and polymer >=0.4.1 <1.0.0. Let's imagine we're in the future and Polymer 0.5.0 has been released. At this point I'll be presented with a bower resolution asking if I want polymer#0.4.1 or polymer#0.5.0.

I'm personally concerned about this practice of using such a wide version range (>=x.x.x <1.0.0) because it makes it very difficult to freeze a version of your app for production. Hence the [Polymer#0.3.6 tag that went out 11 days ago](https://github.com/Polymer/polymer/releases/tag/0.3.6) because someone complained.

@azakus are there plans to change off of the wide version range to something more restrictive?