Add function to launch an rstudio server instance using docker

Hey @sckott ,

This adds a function `rstudio` based on `do_install` that installs rstudio server using a docker image.  In my tests it works, though it requires you to enter the password a few times.  (`do_install` looks like it might need some fixing since the location of the `status` and `ip` have moved in the `droplets()` return object).  

This configures swap appropriately too so it should work on the smallest droplets.  The default is a basic docker image providing rstudio, but the user can specify an alternate image (like cboettig/ropensci, which has more stuff pre-installed that would otherwise be slow to compile).  In principle this should be easier to maintain.  

One thing I noticed is that if a user hasn't already logged on and changed the password from the emailed one, the interface for updating the password doesn't work in R.  (Presumably that affects `do_install` too).  That's kinda a nuisance since it means you have to know how to ssh from the command-line.