Update docblock comments.

Session is now under Cake\Network namespace.
CakePHP's SessionHandlerInterface has been replaced by the one provided by PHP itself.
Added info about the new "requestCountdown" session config variable.

Refs cakephp/cakephp#2373
Remove flash layout.

Controller::flash() is being removed in cakephp/cakephp#2385. Sync that
change here.
Updated Validation class namespace. Closes #7.