introduce initial, unit tested google cloud platform element set

This is the minimum needed for my codelab (compute engine is the first and only cloud product supported, and only part of GCE at that) but I think it also builds a decent starting point for cloud polymer components. I defined a naming convention for elements and files to hopefully avoid a proliferation of confusing and inconsistent names used across the many cloud products.
This breaks encapsulation a bit. They reach into the main document to find gcloud-gapi/gcloud-config elements. If these elements are used in another element's shadow dom, they won't be found.

Since you're extending from this element, a better way would be to set these properties in the extendee's ready() or have users set them on the outside:

var image = document.querySelector('gcloud-compute-image');
image.gapi = document.querySelector('gcloud-gapi');
image.config = document.querySelector('gcloud-confi');

or pass them in a published properties