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.
Add our GA snippet: https://github.com/GoogleWebComponents/google-signin/blob/master/index.html#L14