get favicon once from getfavicon.appspot.com

get favicon once from getfavicon.appspot.com then store it in the /favicon/ folder
function grab_image & mkdir favicon

please fell free to move this code
Merge branch 'patch-2' of https://github.com/ali0une/kriss_feed
Merge branch 'master' of https://github.com/ali0une/kriss_feed
cache favicon in order not to depend on getfavicon.appspot.com