Rollback awareness.

Tasks are aware if they are in rollbacks; they will be invoked only if
they implement the new interface RollbackAware.

docs/example-config/.mage/tasks/SampleTaskRollbackAware.php

@@ -0,0 +1,19 @@
+<?php
+class Task_SampleTaskRollbackAware
+    extends Mage_Task_TaskAbstract
+    implements Mage_Task_Releases_RollbackAware
+{
+    public function getName()
+    {
+        if ($this->inRollback()) {
+            return 'A Sample Task aware of rollbacks [in rollback]';
+        } else {
+            return 'A Sample Task aware of rollbacks [not in rollback]';
+        }
+    }
+
+    public function run()
+    {
+        return true;
+    }
+}