Adding polling attribute.

Hi,

I started working on a new leisure project of mine with Polymer (what a cool thing) - and I immediately needed polling functionality for AJAX - i.e., I needed an element to update continuously. The only way to do that, I found, was to change the URL or request params with a timeout. Here I just added a poll="{Number} milliseconds" attribute to <core-ajax>. I thought you may find this useful.