Allow components to get the external url of a document

Components can get the relative url of a document such as /xwiki/view/My/Document
but the absolute url such as www.xwiki.org/xwiki/view/My/Document is not available.
This feature would add the accessibility to the documentAccessBridge.
See: http://lists.xwiki.org/pipermail/devs/2010-August/019529.html