Add PubSubHubbub hub urls as feed element

Building a feed reader, with Feedzirra as a core library, over the the last few months has led to using PubSubHubbub extensively to make feed processing more efficient.  This adds a "hubs" element to feeds that parse the rel="hub" links so that PubSubHubbub can be implemented after parsing feeds with Feedzirra.  As issue #123, https://github.com/pauldix/feedzirra/issues/123, discusses, using the add_common_feed_element was not sufficient when there is more than 1 hub and Paul suggested adding it directly to the parsers.

One issue I had is that I cannot find a vanilla, non-feedburner, RSS feed that uses PubSubHubbub, so I just added a attr_accessor, and don't parse it there.

Any questions or feedback welcome.  Thanks!