Added a Cache-Control header

The main goal here is to set the Cache-Control header for any requested image.
This property has been set with these 2 default values:

* **public** it means the cached version can be saved by proxies and other intermediate servers, where everyone can see it.
* **max-age** it lets us say “This file expires in x seconds from now”, which is simpler than setting an explicit date. In our case we set it to 1 year (ie infinity on Internet Time)