GenerateProtobufDefinition fails on interface

I'm trying to find how `onet.Roster` is serialized by protobuf. Sadly, when encountering an interface, such as `kyber.Point`, it fails.
 
```golang
protobuf.GenerateProtobufDefinition(os.Stdout, []interface{}{
            new(onet.Roster),
}, nil, nil)
```

It would be nice to support it, such as using the `Any` prototype defined in proto3, or simply a `reserved` with a comment.