Parse affiliations in 2014 NLM DTD

NLM changed the structure of the Medline DTD in 2014 (see
http://www.nlm.nih.gov/bsd/licensee/announce/2013.html#d08_13
for a summary). Most notably, affiliations will be reported
for all authors, leading to the `Affiliation` element
being moved to the `Author` elment.

No other changes should be required; other 2014 changes like
the new publication types have been integrated already. In 2014
the Databank element's behavior has been changed but it isn't
yet being used in the hypothesis_graph package.