Allow fetching fragment html from different URL

This allows you to optimize network traffic and client side processing on large static pages.
You can give a fragmentUrl property to fetch the fragment content from different URL than the link URL. The link URL is still displayed as the URL to the user.

Also contains change on the test server behavior concerning the addition of X-PJAX-URL header. This header is needed only when using redirect and thus it's now only given on redirect. This way pjax logic is tested by default without the servers redirect command in X-PJAX-URL.