Create a Transformation to check broken external links

This is useful when you administer a wiki and wish to find out all pages containing broken links.

Note that the idea is to implement this as a transformation since:

• crawling all pages is time consuming
• when you crawl pages you don't have the user context and thus you can't find out broken links that are generated in scripts

Note that for improve performances the implementation should put links to be checked on a queue and have a separate processing thread + check the same link only every few minutes.