Make HTTP clients only cache graphs as long as we keep them in memcached

We were previously setting Cache-Control: no-cache and Pragma: no-cache
without setting any Expires or Last-Modified headers. This was causing
some clients (e.g., WebKit on iOS) to cache images indefinitely.

We now set Expires, Last-Modified, and Cache-Control: max-age to tell
clients to cache images for as long as we cache them in memcached. If we
aren't using memcached, we tell clients not to cache the images at all.
Merge pull request #978 from aroben/expire-cached-images

Make HTTP clients only cache graphs as long as we keep them in memcached