extend 'twodigitRversion' to handle more types

allow passing of `R_system_version` and `character` class objects to
`twodigitRversion`
add .gitignore and .Rbuildignore
added functions to add pkgs to existing miniCRAN

* added new functions `addPackages.R`:
-- `add.packages.miniCRAN` adds pkgs to existing miniCRAN repo
-- `check.package.versions` reports full filepaths of package files
currently in the miniCRAN repo directory
* added call to `twodigitRversion` into `repoPrefix` where it's needed
* removed call to `twodigitRversion` from `makeRepo`
* added checks to make sure user specified 'path` exists