R wrapper package

[jared321]

Add an R wrapper package to the repository. The repository currently contains a folder R whose contents need to be reviewed and dealt with as part of #6.

Once that is settled, that folder should likely be removed and instead the wrapper should be self-contained within the folder Ropenbt.

[jared321]

As a first step, it's possible that the R package will provide the regression functionality, but not the mixing functionality. This should be clearly documented in the R User Guide, which doesn't yet exist.