Add ruby-additional, elisp files in the CRuby distribution.

- ruby-mode.el is excluded because the one bundled with Emacs 24 should be considered the main line and ruby-additional.el is a patch to make it work like the one in the CRuby repo.
- inf-ruby.el is excluded because there is a better fork in MELPA.
- ruby-electric.el is included because the one in the Ruby repo has its own changes separate from the one in MELPA, which I did not know about when I was working on the former.