 * Remove various imports that cause ImportErrors unless pysvn libs / bzr libs / hbmp are installed

 * Add flunc tests

 * Backtracking on mimetype, moving towards generic metadata 

 * 404 (instead of 500) when visiting a document at a revision past 
   the repo HEAD.  The response body contains a link to the document
   at the repo HEAD.

 * 404 (instead of 500) when visiting a document at a revision prior
   to that document's birth.  The response body contains a link to
   the document at the revision of its birth.
