Namespace registered Faraday middleware

Just namespacing the code for a middleware in a module is, sadly, not enough, as the name when you register it might clash with middleware from other libraries that register the same type of middleware (for example, the [Twitter ruby gem][1] registers a `:parse_json` middleware, which provokes projects using both the twitter and diffbot gems to raise an error when an error response comes back from twitter.

[1]: https://github.com/sferik/twitter