Several notes:
- Rails seems to be a kind of overkill here, I would rather use Sinatra app instead. But the task is the task.
- I thought about wrapping ExchangeRate lib into a gem, but changed my mind in favor of simplicity.
- Local storage. I have choosen the simplest way: file system and YAML format. I understand that it might not work in some environments, such as Heroku with that readonly file system. I could use database instead, but it would me more complicated.
ExchangeRate lib itself can be found in /lib/exchange_rate.rb.