Dispose isolate on V8Js object destruction

This fixes #98 

During runtime all createded V8Object and V8Function instances are tracked,
on V8Js object destruction the isolate is destroyed (and hence the V8Object & V8Function instances become invalid),
Exceptions are thrown as needed if any V8Object/V8Function, that was kept on PHP side, are used after V8Js object destruction

See issue #98 for any more details