Get requests

Yay!  Thanks, @chriddyp -- it's amazing to have the `get_figure()` method! :)
I did some clean-up and played around a bit:
```
install_github("ropensci/plotly", ref="chriddyp-GET-requests")
library(plotly)
py <- plotly()
super <- py$get_figure("mkcor", 118)
super$data[[1]]$y
```