Hook up Javascript implementations to JSAPI class

To enable this to work, developers need to modify the default index.htm generated by Protobuild to have:

        manifests: [
          "MonoGame.Framework",
          "JSILPort.exe"
        ],

This will cause the appropriate MonoGame libraries to load.

If you build a web project referencing MonoGame and you do:

    public static class Program
    {
        public static void Main(string[] args)
        {
            var api = new JSAPI();
            Console.WriteLine(api.Test());
        }
    }

The web page will output "hello" as declared in the Javascript file (even though the C# implementation returns null).