Surface Variant values, at least on Node.cs

[migueldeicaza]

We should at least support a way of getting the value for a Variant.

Those are really boxed types with a limited set of values, perhaps we should surface this as objects?

We could then encode System.Int32 values as VAR_INT for example, and any objects in .NET that do not have an equivalent could be GCHandle wrappers to the boxed value.

[EgorBo]

CxxBinder already supports Variant (as an arument only, it needs a fix for "Variant" as a return type).
It generates overloads for all possible types that can be used as Variant
https://github.com/xamarin/urho/blob/master/SharpieBinder/CxxBinder.cs#L1325