special case 'R-mode' treated during conversion from source to org and from
org to source ('R-mode' returns 'ess-mode' as major-mode, but 'ess-mode' gives
an error when used with 'funcall').
special case `R-mode' treated when wrapping source in source-blocks during
conversion from source to org (use "R" instead of the returned "ess" as
org-babel-language).
Made outorg significantly smarter about blocks.

Now, only source-blocks for the major-mode language of the source-code buffer
are treated as code (or example blocks, if the major-mode language is not
found in `org-babel-load-languages') are treated as code. All other blocks are
treated as text, thus appear outcommented in the source-code buffer, but ready
for evaluation and export in the *outorg-edit-buffer*.