Scale datetime domain approriately

When domains are of type datetime, the `unlist` operation automatically coerce the values into timestamps in seconds. Using `format_vec_d3json` to proactively cast datatype before range calculation.

Quick example to reproduce the problem:
```
library(lubridate)
view_dynamic(data.frame(date=ymd(20140101)+months(1:5), value=runif(5)) %>% 
ggvis(~date, ~value) %>% 
layer_points() %>% 
set_dscale("x", "datetime", nice="month"))
```

Let me know if you have any suggestions or concerns. Also I can probably get CLA back to you guys by Friday.