Add some project files for working in RStudio

[ghost]

It might be nice to add some project files so that things like indentation settings, whitespace settings, etc. are standardized for all the people working on the codebase. Not sure what particular options are your preference, most of this was auto-generated by RStudio.

[joshuaulrich]

I don't use RStudio, which is why I didn't include this commit in your prior pull request (#3). I understand that it would be nice to have standardized settings, but the files required for that are dependent on IDE/editor choice.

[ghost]

If they don't get in the way, it might be good to include them anyway. Besides helping people who do use RStudio, it also gives a very readable list of how the package is expected to be developed - failing that, it would be good to write up a README or HACKING file or whatever, to indicate which conventions you're following.

If you don't want the pack.Rproj file, it seems like at least the .gitignore and .Rbuildignore files would still be appropriate?

It's probably best that it wasn't merged in the previous request anyway, it's really a separate issue.

[joshuaulrich]

Merged into develop in commit ff59f4e.