Merge pull request #7 from jagregory/entry-xmlns

Add optional xmlns to AtomEntry
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.

  [1]: http://tools.ietf.org/html/rfc4287#section-4.1.1