Added a tool to enable platform specific code

Implemented and tested the static utility On, which allows code to be
executed based on Device.OS and Device.Idiom with the following syntax:

On.iOS(() => {
//Do something unique to iOS
});

The project was refactored into application directories, and a test
project was for GraphBuilder.

As noted in the remarks, this class is over-engineered for the purposes
of this project. Still, it served as a useful way to explore creating
and testing PCL libraries.

The On class also allows this project to be extended to support other
devices with some of the benefits of SAP projects. It was a good
learning experience for me.