easier feed extension

It's a common practice when handling atom feed extension as Bill De Hora defends, that we use a custom namespace for our own values. In this situation, describe_collection and describe_member would help a lot if they already provided a custom namespace as in:

describe_collection(@basket.selected_items, :namespace => "xpto") do |collection, ns|

and still support

describe_collection(@basket.selected_items) do |collection|