'Convenience method that provides a quick way to add a canonical user
grant to a key. This method retrieves the current ACL, creates a new
grant based on the parameters passed in, adds that grant to the ACL and
then PUT\'s the new ACL back to GS.
:type permission: string
:param permission: The permission being granted. Should be one of:
READ|FULL_CONTROL
See http://code.google.com/apis/storage/docs/developer-guide.html#authorization
for more details on permissions.
:type user_id: string
:param user_id: The canonical user id associated with the GS account to
which you are granting the permission.'

def add_user_grant(self, permission, user_id):

    acl = self.get_acl()
    acl.add_user_grant(permission, user_id)
    self.set_acl(acl)
