Content-disposition header for friendlier "save image as..." from browser

Hi there,

Your SVG fix was great. However, when right-clicking to save the image, the default filename in the save dialog was "index.php." I've added a content-disposition header to rename the file so it has a SVG extension instead. It also saves the file with the same name as the cachegrind file, which may be a nice bonus.

I had some indentation issues so please excuse the extra line changes in the diff. I'm not really handy enough with git yet to fix my mess. :(