RuntimeTypeModel exposed as public & lazy

Changed RuntimeTypeModel to be public. It is common in ProtoBuf usage to precompile types, which was not possible with the previous solution. Now you can something like that from for example global config:

            ProtoBufFormatter.Model.Add(typeof(MyType),false);
            ProtoBufFormatter.Model.Compile();

Additionally made it Lazy so that the formatter can be added to formatters collection without much footprint. Also removed dependency on JSON.NET (not used in this project at all).