Add ajax and image based ping locations for cached hosts.

[CaptainN]

If you are hosting on WP Engine or using WP Super Cache, the session cookie will not work from all pages, if the user is not logged in, since the cache would serve a static version of the page and never activate the session handler.

This creates a problem where the cookie can expire, and/or never be set. One way around this problem is to use an ajax handler (which requires JavaScirpt) or a linked image (need to verify that will work - this does not require JavaScript), which avoids the cache, and can set and refresh the session cookie.