Add fee endpoint

a) Because hardly anyone exports things at the top this way. It's confusing.
b) We don't define exports that way in all other files

+ https://github.com/ripple/ripple-rest/blob/develop/api/settings.js
+ https://github.com/ripple/ripple-rest/blob/develop/lib/errors.js
+ https://github.com/ripple/ripple-rest/blob/develop/lib/remote.js
+ https://github.com/ripple/ripple-rest/blob/develop/lib/router.js

etc