Make Rcpp header only

[romainfrancois]

We can't really achieve header only, but we can achieve that what the client package sees is only headers. So that we no longer need linking against the Rcpp library, which makes this library useless.

I did this in Rcpp11

[romainfrancois]

Some notes here. https://github.com/RcppCore/Rcpp/wiki/header-only-Rcpp

I'm almost there and will merge this into master once travis runs the tests happily.