add_annotations should work in 3D

That is, this should 'just work':

```r
plot_ly(mtcars, x = ~wt, y = ~mpg, z = ~disp) %>%
    add_markers() %>%
    add_annotations(text = row.names(mtcars)) 
```

The tricky bit here is that the annotation array needs to be placed under `layout.scene.annotations`, not `layout.annotations`
