Allow apps to configure faraday logger

This code in haystack reaches into elastomer-client's Faraday object to set the logger:

```
if Rails.env.development?
Haystack.elastomer_client.connection.response :logger, Rails.logger
end
```

https://github.com/github/haystack/blob/master/config/initializers/elasticsearch.rb#L18-L20

This shouldn't be necessary. Elastomer::Client should provide a method to set a logger object.

Start with Elastomer::Client and the connection setup used by the Haystack initializer at config/initializers/elasticsearch.rb#L18-L20. Trace how the Faraday connection is exposed and determine where a logger-setting method belongs. Done means an app can configure the logger through Elastomer::Client without reaching into the Faraday object.

索引モデルが issue の本文から書いたものです。