Add Faraday Adapter

An implementation of the rack-client adapter for faraday described here: lostisland/faraday#244

For applications or libraries that use faraday and allow for a configurable adapter, this adds support for injecting a rack-client stack into the faraday client configuration. For example, if I wanted to test my rails app directly from my client app:

``` ruby
MyClient.new do |faraday_builder|
  faraday_builder.adapter :rack_client do |rack_builder|
    rack_builder.use Rack::Lint
    rack_builder.run MyApp.new
  end
end
```