Added platform-specific logic handling

Implemented and tested the static utility class 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
});

This provides similar functionality to one of the Device.OnPlatform
overloads, but extends the Device APIs functionality to provide a
clean syntax for code that handles a special case on one platform.

The On class was implemented to be testable in a platform agnostic
manner using the [Conditional("DEBUG")] attribute. The functionality
provided by Device.OnPlatform and my On adequately supplement many
of the benefits of a SAP project, but not all of them.

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 functionality.
Merge branch 'master' of https://github.com/drubino/GraphBuilder