.gitignore for ignoring miscellaneous existing directories on OBF server
Remove dead link "Japanese Wiki page" (http://dev.bioruby.org/ja/)
Change ChemRuby link to RubyForge because chemruby.org is lost.
Remove old announcements: about RubyForge BTS and about IRC conference
Improve "Overview" contributed by Pjotr Prins.
Remove old video cast link and add links to recent presentations