Allow use as gem on Rails 2

This pull request allows Rails 2 apps that are using bundler to pull in `attachment_fu` as a gem.

Backstory: I'm attempting to upgrade a legacy Rails 2.3 app, and I'm first upgrading all dependencies to the most recent version possible. This way there's less to deal with when the actual upgrade happens. Being able to have `attachment_fu` already up to date and in the Gemfile would be helpful. I suspect that many users of `attachment_fu` are (or will be) in similar situations.

I've left the gemspec version and CHANGELOG alone (I usually leave that up to the maintainer), but I'm happy to update with those changes the PR if that would help.

Thanks!