Memory fix

- This fixes a memory leak created by a loop in the retain cycle between the MGCommandGroup/MGSequentialCommandGroup & MGCommandExecutor

- It was developed using XCode 5, iOS SDK 7.0

- It was tested in XCode 5, but also XCode 4.6.3 to preserve downwards compatibility 

- Contributions were made by Daniel Doubleday (https://github.com/doubleday) and me