load resources with query offered by platform.js

Normally, there would be version number for `platform.js`. And there always be version number in the home page of `http://www.polymer-project.org/`.

This patch and other patch in `HTMLImport` add the version to the query of resources, make it easier for developer to manage the url.

Usage:

        <script src="/platform.js?2014xxxx"></script>

Then, all the link with import, and css will be requested added the vesion in the platform.js.