Added UIDynamics example

Added UIDynamics example for adding gravity effect. UIDynamics is
available to iOS 7 and above, so project target is now iOS 7.
Revert "Added UIDynamics example"

This reverts commit e68f725aad70590ea71a35bcfeb0e09a62e1be21.
Revert "Revert "Added UIDynamics example""

This reverts commit 3caa88db29b76a232063e509b5c6aff8dbe13075.
Added UIDynamics example

UIDynamics is available in iOS 7 or higher.