[dev] Permissions cache

[Duck]

I was trying to find out a betters solution for the permission system today. 
While I was trying to figure out possible solutions, one thing come to my 
mind. Why we don't cache the permission data? Permission data generally don't 
change some often. As I see in the applications usages it will be enough to 
cache the getPermission() and exists() methods. Even solutions like phpGACL 
cache this data.

Duck