Exposed protobuf RuntimeTypeModel as public

Public static *RuntimeTypeModel* allows the developer to use precompilation of types and set
things like *UseImplicitZeroDefaults*. Up till now we were using *Serialize.NonGeneric* which defaults to *RuntimeTypeModel.Default* which is a bit limiting.

For example:

    ProtoBufFormat.Model.UseImplicitZeroDefaults = false;
    ProtoBufFormat.Model.Add(typeof(MyType),false);
    ProtoBufFormat.Model.Compile();