Allow scripts to easily know the reference of the document that contain them

When writing a script, you sometimes want to refer the document containing that script, and this could be not the same as the current document (currently $doc).

So it would be nice to have the same easy binding to this container document, and this would work for all kind of scripts, like JSX, SSX, macro, wiki components, etc...

This would allow to write stuffs like:
$xwiki.jsx.use($self)
url($self.getAttachementURL('image.png'))
$self.getObject($self.getDocumentReference())
...