Rackspace demo config

I thought this might be helpful, because I didn't come across any Rackspace examples in either this project or in Paperclip itself.  (There is an example Fog config on Paperclip, but it uses the local provider.)

This can also be easily deployed to Heroku.  I didn't update the README, but the steps are basically the same, except that you need to set the rails env (since I left production.rb as S3), and you need to switch AWS variables for Rackspace variables:

```bash
heroku create
git push heroku master
heroku rake db:migrate
heroku config:set RACK_ENV=rackspace RAILS_ENV=rackspace
heroku config:set RAX_USERNAME=your_rackspace_username
heroku config:set RAX_API_KEY=your_rackspace_api_key
```