switched to debian:7.6 since wheezy keeps getting updated, and as such m...

switched to deiban:7.6 since wheezy keeps getting updated, and as such makes problems with the debian snapshot

added example R script for installing from previous CRAN states with swi...

since this was fixed in: 
https://github.com/rocker-org/rocker-versioned/commit/16812892cec5f606a39554e8d5e792c36278f0e8

first proof of concept of MRAN repos, and some work-arounds

work-around for problems illustrated by 2.12.0-repo dir using the snapshot as extra source + an APT preference file setting higher priority for r-* deb packages from the snapshot

configured apt for the debian snapshot corresponding to R-2.15.1-4