Instance protected plugin correctly and modify default sanbox value

plugin/buy_courses/database.php

@@ -5,7 +5,7 @@
  * Date: 21/05/14
  * Time: 12:19 PM
  */
-$objPlugin = new Buy_CoursesPlugin();
+$objPlugin = Buy_CoursesPlugin::create();
 
 $table = Database::get_main_table(TABLE_BUY_COURSE);
 $sql = "CREATE TABLE IF NOT EXISTS $table (
@@ -295,7 +295,7 @@ Database::query($sql);
 $table = Database::get_main_table(TABLE_BUY_COURSE_PAYPAL);
 $sql = "CREATE TABLE IF NOT EXISTS $table (
     id INT unsigned NOT NULL auto_increment PRIMARY KEY,
-    sandbox VARCHAR(5) NOT NULL DEFAULT 'SI',
+    sandbox VARCHAR(5) NOT NULL DEFAULT 'YES',
     username VARCHAR(100) NOT NULL DEFAULT '',
     password VARCHAR(100) NOT NULL DEFAULT '',
     signature VARCHAR(100) NOT NULL DEFAULT '')";

plugin/buy_courses/src/buy_course_plugin.class.php

@@ -18,7 +18,7 @@ class Buy_CoursesPlugin extends Plugin
         return $result ? $result : $result = new self();
     }
 
-    public function __construct()
+    protected function __construct()
     {
         parent::__construct('1.0', 'Jose Angel Ruiz, Francis Gonzales', array('paypal_enable' => 'boolean', 'transference_enable' => 'boolean', 'unregistered_users_enable' => 'boolean'));
     }

plugin/ticket/database.php

@@ -3,7 +3,7 @@
  * Contains the SQL for the tickets management plugin database structure
  */
 
-$objPlugin = new TicketPlugin();
+$objPlugin = TicketPlugin::create();
 
 $table = Database::get_main_table(TABLE_TICKET_ASSIGNED_LOG);
 $sql = "CREATE TABLE IF NOT EXISTS ".$table." (