Add IB_DESIGNABLE and IBInspectable directives to ITSwitch

This allows you to preview and edit the appearance of an ITSwitch in Xcode 6's IB. The shadow gets clipped because of the way that IB_DESIGNABLE views are rendered, but that seems reasonable.

![screenshot 2014-11-12 22 57 21](https://cloud.githubusercontent.com/assets/594059/5023958/46743abe-6abf-11e4-928f-d336e5ea53ff.png)