Add support for Guid types

[Michannne]

Creating a model such as the following:

    public class Book_WithValueTypes : Book
    {
        public Guid Identifier { get; set; }
        public char Category { get; set; }
        public decimal Dewey { get; set; }
        public byte OwnerCount { get; set; }
        public sbyte Modifier { get; set; }
        public short Value { get; set; }
        public uint Copies { get; set; }
        public ulong Pages { get; set; }
        public ushort AuthorCount { get; set; }
        public BookDescription Info { get; set; }
    }

fails to create Object node as Guid is not supported