Add DrushTask (CommandStack)

Working with drush recently, and implemented the commands I used most.

Example:

 ``` php
 $this->taskDrushStack()
     ->drupalRootDirectory('/var/www/html/some-site')
     ->uri('my-multi-site')
     ->maintenanceOn()
     ->updateDb()
     ->revertAllFeatures()
     ->maintenanceOff()
     ->run();
 ```