Add vanilla "comments" Rails 3 application.

Whereas the current "poll" Rails application runs on Rails 2, "comments" runs
on Rails 3. There are numerous other differences, such as using mysql instead
of postgresql, and using Ruby 2.0.0-p0 instead of 1.9.3-p0.
"comments" app uses mysql for backend.

By default, Rails uses SQLite as a database backend. This change forces mysql to
be used instead, which allows for more real-world testing. And it ain't hard to
create a database and authorized user in mysql. Yay!