Update Rcpp.rmd

Hadley.  You deserve to be knighted for all that you have done for R.  Thanks so much.  

I just noticed that if you pull the division by n outside of the for loop, you get about an 8-fold 
improvement in speed over the vectorized R operation (and about 6x faster than meanC), as division is much costlier than addition or multiplication (if I remember correctly).  

I did that and made a meanC2 function and got this:

```
Unit: microseconds
      expr     min      lq   median       uq      max neval
   mean(x) 862.276 862.773 862.9605 863.3390 1071.375   100
  meanC(x) 775.445 775.896 776.5775 777.1715  954.236   100
 meanC2(x) 119.946 120.176 120.4670 121.1135  245.384   100
```

Your demonstration of how to micro benchmark c code here is fantastic.  It really has helped me get 
my head around when I should (and shouldn't) bother implementing something in compiled code.

I can't stress enough how helpful this book is.  Thanks again.

eric