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 [Diffbot ruby client][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/diffbot/diffbot-ruby-client
Merge pull request #610 from foca/namespace-faraday-middleware

Namespace registered Faraday middleware