Update notation and naming for modern use

* Changed parameter name from `base.url` to `base_url` because the `.` notation is ambiguous (http://kbroman.org/hipsteR/).  Indeed, you could now run `plotly(base.url="https://plot.ly")` as well as `plotly(base="https://plot.ly")`, which is *so* scary, right?

* Renamed R file `one_to_one_corr.R` into `corresp_one_one.R` so it comes before `ggplotly.R` in alphabetical order--and we can dispose of the `Collate` field in `DESCRIPTION` and we can thus make use of modern tools (I mean, `roxygen2` and `devtools`).
@jdblischak Following up on our R packaging conversation, see above, if you're curious. 

Will update `NEWS` and `DESCRIPTION` after review.

Thanks @chriddyp