allow setting qrng as the user-define RNG for R

added function setup_QRNG("user", "pw") that installs
the QRNG as default RNG in R, i.e. subsequent calls to runif
or anything else will use this RNG, instead of R's own. See
also ?Random.user for details.
Merge branch 'master' of github.com:edzer/qrng