Fix regexps for replacing Org-mode entities.

Hey!

This code modifies the replacement of Org-mode entities. First, I don't check regexps for all entities, which should take a much shorter time that it was before. I took proper regexp from original `org.el` (see function `org-fontify-entities`). Second, no need to close every Org-mode entity by `{}`, instead, the default Org-mode behavior is used. I re-created file `gen-special-replace.el` to automatically generate files for the replacement of Org-mode entities. Can you create an example for Org-mode entities to ensure that the code works right?

**P.S.** I'm looking forward a new release, but I would like to add some code before that. I wish to use direct HTML code with lines `#+html:HTML_code`. In addition, maybe I'll take a look at issue #30 and some other improvements. In any way, I'll tell you if I realize these ideas.

Best regards,
Vladimir.