Add caging area

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:

```objective-c
[view enableDragging];
CGRect cagingArea = <some CGRect>;
view.cagingArea = theCagingArea;
```

and to disable the caging functionality, set the `cagingArea` to `CGRectZero`:

```objective-c
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.

![caging](https://cloud.githubusercontent.com/assets/879038/4800843/6c1eaf70-5e2a-11e4-88d0-a355670f5c9c.gif)