Set site.path to context path, prepend to page urls

Calculate the context path (as we know it in the Java world) from the site.base_url and prepend it to the page urls. This allows an Awestruct site to work when running under a sub-path of the domain.

The page urls don't need the full base url since they are already served from that domain. The context path just ensures that the relative references (from one page to another) are correct.