Improve custom configuration

1. Hashes can be assigned
2. We don't need a special level anymore

The method chain only works in the top level.

If users need a second level they need to assign a OrderedOptions to the
key:

    config.resque.server = ActiveSupport::OrderedOptions.new
    config.resque.server.url = "http://localhost"
    config.resque.server.port = 3000