imagecache_create_url to include creation time of the file to prevent browser caching

Many browsers will cache the image file on the local host. For instance when new profile picture is uploaded, browser might display the old version. Same goes with other content images as well.

This patch adds timestamp as a parameter to each urls created. It is similar as giving the $bypass_browser_cache parameter value 'true', but more efficient. Creation time does not change unless the file has been changed.