Improve detection of org library directory

* lisp/ob-R.el (org-babel-R-directory-in-org): replace split-string
  with usage of emacs file name manipulation functions.
Moving org:functions env management into variables

* lisp/ob-R.el (org-babel-R-variable-environment-name): Moving R
  functions to create `org:functions` and to load .R files from
  `ORG/etc/R` into functions. The name of the environment
  `org:functions` is hard-coded due to simplicity.
Usage of .org file for documentation and source

* etc/R/org_functions.R: Changed from actually edited file to tangled
  from org_functions.org

* etc/R/org_functions.org: Source file to be tangled to generate
  org_functions.R. Contains documentation of the functions.

* lisp/ob-R.el: Minor formating