Namespace config options passed to Client

Instead of having all the configuration options mixed together we
namespace the options that will be passed on to the
`Elasticsearch::Transport::Client` under the `client` namespace.

```ruby
Elasticsearch::Documents.configure do |config|
  config.index_name     = 'my_index'
  config.mappings       = my_mappings
  config.client.url     = 'http://localhost:9200'
  config.client.logger  = Rails.logger
  config.client.adapter = :custom
end
```