Added the bounding property to sprites

Added the bounding property to sprites. See design document here:
https://docs.google.com/document/d/16xhZUdV-r6Ic7YgMAwFV7B6B5C0twt-J14ZaQwKmsAQ/edit?usp=sharing

You can find a version of this application running here:
http://1.appinventor-boundedsprite.appspot.com/