fix packaging

[bgoodri]

I'm not sure why, but these commits allow your package to call rstan::sampling in parallel. It has something to do with putting methods into Depends: in the DESCRIPTION file and putting import(Rcpp) into the bDif.R file.
Also, you should be able to just put rstan into Imports: rather than Depends:.

[stephensrmmartin]

Thanks for this. I think the only issue was adding Rcpp to the namespace imports; once I added Rcpp to the namespace imports, it worked.

As for whether to keep rstan in imports or depends, I chose depends because I think the user would benefit from all the rstan summary and plotting functions, and Depends automatically attaches the rstan package for that purpose. Is that a bad idea?

And finally, I thought methods was no longer required importing, but perhaps I'm wrong (I thought as of some recent R version, methods is assumed attached, but maybe I'm wrong).