Add support for UNC paths

A significant usability issue for Windows users: Pathnames contain backslashes (). XWiki uses the backslash as an escape character. So UNC paths like \\myserver\myshare\mydoc.txt aren't rendered properly in an XWiki document.

A workaround solution is to wrap the path in {pre} .. {/pre}. But it would be preferable if XWiki recognized UNC paths and rendered them properly.

Consider adding support for recognizing UNC paths like \\myserver\myshare\mydoc.txt, and making them into hyperlinked URL's like this: file://///myserver/myshare/mydoc.txt (correctly handled by both IE and Firefox).