[wicked] link to attached files

[Ben Chavet]

The attached patch allows for linking to attached files (not just images).  It
looks for [[file filename]] tags in the wiki text.

I know we are planning on moving away from attachments, but I think these
classes are a good first step in that direction.

--Ben

New Files:
  wicked/lib/Text_Wiki/Parse/File.php
  wicked/lib/Text_Wiki/Render/Xhtml/File.php

Modified File:
  wicked/lib/Page.php