Override Gravatar.com image caching.

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Gravatar.com is a big threat to privacy. Through the gravatars images it has trackers on every blog I read.
And Gravatar use a ridiculous caching time to exploit that: 5 minutes.

Here is a excerpt of a HTTP request for a gravatar:

Date: Fri, 23 Aug 2013 22:04:17 GMT
Last-Modified: Wed, 01 Aug 2012 15:35:21 GMT
Expires: Fri, 23 Aug 2013 22:09:17 GMT
Cache-Control: max-age=300

Still I like Gravatar images. But most Gravatar users only set their profile image once in a lifetime.
So I would like that Ghostery override header return by gravatar.com to instead cache imges for 6 months.