The page demonstrates four example uses of the transformer.js library. All of them have rectangles as transformables in them. Use drag and drop with your mouse to move them around, the mouse scroll wheel to scale them, and ctrl key + mouse scroll wheel to rotate them.
The Hammer.js example below also allows for touch input, so you can use it on any touch-enabled device (e.g., smartphone or tablet) to pan, rotate, and zoom a transformable.
At the moment, there is no documentation of the code. This will follow. For now, you can inspect the source code to get an idea of how to use the transformer.js.