Add caging area

[jrmsklar]

This pull request adds the functionality a caging area for the draggable view, in which the view can not be dragged out of.

To add a caging area, set the cagingArea property on the view:

[view enableDragging];
CGRect cagingArea = <some CGRect>;
view.cagingArea = theCagingArea;

and to disable the caging functionality, set the cagingArea to CGRectZero:

view.cagingArea = CGRectZero;

This also adds a UISwitch to the demo project to showcase this behavior. In this demo, the caging area is the frame of the FPViewController's view.

[andreamazz]

Hi @jrmsklar
Thanks for your contribution, great job 👍