Fix for #937

As I mentioned in bolt/bolt@0640bca81d16361630ff326cadc9d47a3125b932 I encountered some errors. I figured out the url check for thumbnails is not made properly. The REQUEST_URI variable is not started with /thumbs when Bolt is running from a subdirectory. I also used timthumb.php-s check for prober url format beacuse I think we don't want malformed thumbnail url-s to redirected to timthumb.php.
Merge pull request #971 from ntomka/thumb_patch

Fix for #937