stop using reflection in prooduction code

Problem: Reflection is a debugging tool and not really meant for use on production. There are easier ways to check if a class implements an interface with `is_a()`.

Solution: Get rid of reflection use and switch to `is_a()` in TaskManager, refactor the Enum thing that does not make sense since it is not being used.

The `is_a()` solution is really straightforward and has been supported in php for ages.

The Enum thing was a copy paste hack from stackoverflow and ignored the simple solution mentioned there while not using the features of the advanced one.

airtime_mvc/application/common/TaskManager.php

@@ -350,8 +350,7 @@ class TaskFactory {
      * @return bool true if the class $c implements AirtimeTask
      */
     private static function _isTask($c) {
-        $reflect = new ReflectionClass($c);
-        return $reflect->implementsInterface('AirtimeTask');
+        return is_a('AirtimeTask', $c, true);
     }
 
     /**