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.