Fix/license for cran

I'm not certain that this is the right incantation but taking a bit of inspiration from e.g. 

<http://cran.rstudio.com/web/packages/optparse/COPYRIGHTS>

I can offer this small patch to get things rolling again.
I think I preferred the explicitness of the original license file.  @jefferis can you please restore?

@gaborcsardi maybe, but I think a CLA just adds more hassle.