Added ecwid_ajax_indexing.php

The file contains everything necessary for making an Ecwid storefront
comply with Google AJAX indexing specification
(https://developers.google.com/webmasters/ajax-crawling/)

More details:
http://kb.ecwid.com/w/page/62638683/Google%20AJAX%20indexing%20for%20you
r%20Ecwid%20store