Set max-age header to Amazon S3 resources

Hey guys,

This improvement is made becuase we are not sending any cache header on amazon s3 resource. Causing several conditional request unnecessarily.

In order to do this I added a configuration entry that enables the user to decide to use this option. 

Nowadays the browser caches those images based on some heuristics
http://blog.httpwatch.com/2008/10/15/two-important-differences-between-firefox-and-ie-caching/

What do you think?