Introduce new Resource module to represent a Resource being addressed

The idea is to rename the URL module's XWikiURL, AbstractXWikiURL, XWikiEntityURL, etc into Resource, AbstractResource and EntityResource and move them to a resource module.

The reason is that they are not URL and are thus badly named. In addition since the goal is to the resource in the Execution Context and the EC should be independent of any environment it's not right to put a URL in it. A Resource is much better.

This also brings us one step closer to http://markmail.org/thread/wfj4dupq76agqonz