More tweaks to RStudio autocompletion + other minor changes

- Autocomplete `row.names` for `data.frame`s in e.g `mtcars[`
- Don't provide argument completions for `c()`, `list()` (since the appropriate completions almost always come from an outer scope, e.g. `mtcars[, c(|]`.
- Add a proxy icon -- still need to determine what we really want the behaviour to be with it, but it does now afford us a couple little things (looks nice; double clicking it maximizes the window; but dragging doesn't do anything sensible...)

![screen shot 2014-12-17 at 12 46 52 pm](https://cloud.githubusercontent.com/assets/1976582/5478664/01936ffe-85eb-11e4-9455-4f97a171f5e0.png)