Add function use_appveyor() similar to use_travis()

[AppVeyor](http://appveyor.com) is a Windows-based CI system much like Travis CI, and also free for open-source projects. I have prepared [a wrapper around r-travis](/krlmlr/r-appveyor) so that AppVeyor can be used to test R packages (and also later deploy them in binary form). It's work in progress, but I think the format of the `appveyor.yml` configuration file is more or less stable.

Example: [Test output](https://ci.appveyor.com/project/krlmlr/rappdirs/build/1.0.77) for a build of the `rappdirs` package.

Pull request follows.
Since basically every devtools function starts with `pkg`, you don't need to explicitly name it.  I think it's also better to preserve the user friendly "Adding Rstudio project file" rather than just "Adding xxx.Rproj".