Add support for Rails 4.1 secrets.

This is a first spike for that @josevalim proposed on #2821. Upgrade existing apps will be simple - developers just need to move their keys to the `secrets.yml` - but on fresh apps its required to add the `secret_key` key manually. We could still generate a token inside the initializer as a fallback (so the app can boot without this step) or figure out an automatic way of placing the token on the application configuration.