Allow multiple links in atom feeds and entries

As per the Atom spec, [Section 4.1.1][1], it's possible to have multiple links for feeds and entries with varying rel values. For example: self, current, prev-archive, and next-archive.

This change adds an extra `Links` collection to the `AtomEntry` and `AtomFeed` structures. I left the existing `Link` in there for compatibility, but I'd prefer to remove it if possible. The changes are only applied to the Atom structures, and not RSS (or Feed/Item) due to RSS not using more than a single link.

  [1]: http://tools.ietf.org/html/rfc4287#section-4.1.1