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).
Actually, I had already done that:

```
// The next line is all it takes to find the bigmemory
// headers -- thanks to the magic of Rcpp attributes, 
// and as bigmemory now accesses Boost headers from the BH package,
// we need to make sure we do so as well in this Rcpp::depends comment.
//
// [[Rcpp::depends(BH, bigmemory)]]
```