moved examples back to install_mran, rrt_deps, and rrt_refresh, roxygenise/document was failing with ref to inst/examples
fixed broken mran_diffs fxn due to changes in server paths
mran_snaps gains param date, and mran_pkg_metadata now uses mran_snaps internally so user only has to supply a date
changed mran_pkg_avail name to mran_pkg_versions to match more appropriately what fxn does
changed mran_pkg_avail name to mran_pkg_versions to match more appropriately what fxn does
added new helper fxn to construct a url into mran_pkg_metadata and mran_pkg_versions
changed mran_pkg_avail fxn name to mran_pkg_versions
fixed install_mran, should be working smoothly now, required small changes to pkgs_mran, closes #45