Abstract Permission Manager checkPermission operation

Abstract Permission Manager checkPermission operation, add this method to the API
This accomplishes two things:
1. makes it easier for Jetspeed developers to use the Jetspeed API to do a permission check
it does not suit all cases, for there are times when you want to actually excecute the privileged code
but it does make for a quick security check
2. for permission managers not using Java security policies, this abstraction will be a necessary addition to the Jetspeed API
to check portlet permissions