Added optional dismiss link for admin notices

Usage:

$this->admin_notices[] = array(
    'type'    => 'error',           // 'error' (red) or 'updated' (green)
    'msg'     => 'Message goes here.',
    'id'      => 'unique_id',       // Use a string that represents the error or notice
    'dismiss' => true,              // Set true to include a dismiss link, false otherwise.
);