Attached is a small patch for a small issue.

Sometimes a language uses a major mode which can't be guessed from it's
name.  This patch introduces the `org-src-lang-modes' variable which can
be used to map language names to major modes when this is the case.
This is used when editing a source-code block, or when exporting
fontified source-code with htmlize.

So far the only instance of this that I know of is ocaml and
tuareg-mode, so that's the only thing that `org-src-lang-modes' is
pre-populated with.  Maybe there are other instances as well?

Thanks -- Eric