Change sitemap-check to File-Check

[peterukena]

Just because I included the robots.txt check to the sitemap-check-model, I thought of using it to check for specific files and their last_modified date.
That could include:

• any sitemap
• htaccess
• wsdl file (api.php)
• robots.txt

I need to think of a nice way, to tell the new check, which files to check for. Config XML is out, because Sitemaps are fetched from magento and the other files are kinda hardcoded - that would require two different XML configs which is too complex.
I am thinking of a hardcoded array for start and make it configurable via backend (checkboxes) late, when we start to build up system.xml and a config-model maybe.