Update 2013-03-14-using-bigmemory-with-rcpp.md

Added the BH dependency to the Rcpp header (required to compile successfully), and added a note about package development (you need C++11 as a SystemRequirement to compile without warnings).
i)  Try updating and see what I just committed. Now builds without warnings (squashed another one about int / unsigned int)

ii)  You are confusing the C++11 language standard (which Rcpp has supported for years, I wrote Rcpp Gallery articles 1 1/2 years ago) with the Rcpp11 project.  This gallery is about Rcpp which allows you to use C++11 or any other C++ feature or dialect without imposing one -- in clear difference to Rcpp11 which imposes one for its internals.