Add a RackClient Adapter

This adds [rack-client](https://github.com/halorgium/rack-client) as a possible adapter for Faraday to use. This allows Faraday to rack up any rack middleware that is compliant with the rack spec:

``` ruby
Faraday.new do |conn|
  conn.adapter :rack_client do |rack_builder|
    rack_builder.use Rack::Lint
    rack_builder.run MyRailsApp.new
  end
end
```

The example above would run every request generated by faraday (and every response generated by the rails app) through rack-lint.