Added SessionHelper to plugin

Add 'Session' => array('className'=>'BoostCake.BoostCakeSession'), to public $helpers in Appcontroller.
Usage:
$this->Session->setFlash('Message', 'alert');
$this->Session->setFlash('Message', 'info');
$this->Session->setFlash('Message', 'error');
$this->Session->setFlash('Message', 'success');

info alert is the same as default so you can use $this->Session->setFlash('Message'); for default or info elements