Adding an option to silence Sidekiq startup logger

Allowing to use
``` Ruby
Sidekiq.configure_client do |config|
  config.options[:silent_startup_logger] = Rails.env == 'development'
  config.redis = {url: 'redis://127.0.0.1:6379', namespace: 'my_app'}
end
```

To silent the following output in development:
```
2014-06-22T01:28:11Z 14217 TID-oqy477vo8 INFO: Sidekiq client with redis options {:url=>"redis://127.0.0.1:6379", :namespace=>"my_app"}
```