added norm(), Im(), Re(), cbind();
fixed rbind(), ":"();
moved "using namespace" to header;
replaced Matrix::norm(A, "1") by norm(A, "1");
added RcppKalman::expm() to benchmark;
matrix sizes are now 5, 50 and 500 to get reasonable bench times;
increased threshold in diff() between higham and ward expm();
Merge branch 'master' of https://github.com/sgsokol/rex2arma