roup Polymer Core Elements

The `core-ajax` element exposes `XMLHttpRequest` functionality.

    <core-ajax
        auto
        url="http://gdata.youtube.com/feeds/api/videos/"
        params='{"alt":"json", "q":"chrome"}'
        handleAs="json"
        on-core-response="{{handleResponse}}"></core-ajax>

With `auto` set to `true`, the element performs a request whenever
its `url` or `params` properties are changed.

Note: The `params` attribute must be double quoted JSON.

You can trigger a request explic