#148 rbind.fill.performance 2

This is a second version for a fix of #148 (first try was #149), targeting an Rcpp implementation of the badly performing nested loop in `rbind.fill`. So far, I have done all the necessary refactoring, the only thing that remains to do is to actually implement the nested loops in Rcpp.

hadley: I will try to implement this against R 2.11.0 and Rcpp 0.8.6 (the most recent Rcpp that is compatible with R 2.11), unless you want to bump R requirements anyway.?