Reference configuration instance instead of class

The current `README.md` code is invalid:

    > Sunspot::Rails::Configuration.solr_url
    NoMethodError: undefined method `solr_url' for Sunspot::Rails::Configuration:Class

I think it's nicer to just see all the configuration:

    > Sunspot::Rails.configuration
    => #<Sunspot::Rails::Configuration:0x007ff707ec8de8 @user_configuration={"solr"=>{"hostname"=>"localhost", "port"=>8983, "log_level"=>"WARNING", "path"=>"/solr/production"}}, @disabled=false, @has_master=false, @hostname="XXXXXXXX", @port=80, @path="XXXXXXXX", @read_timeout=nil, @open_timeout=nil>