Enables the parsing of elements defined in another namespace in Person constructs

[danlyng]

According to the Atom 1.0 spec, foreign markup may be processed as children of atom:entry, atom:feed, or a Person construct. atom-syndication processes extensions in entry and feed but ignores foreign markup in Person elements.

This PR extends the current handling of extensions to Person. I've tried to keep the API consistent, but please let me know if you'd like changes.

Use case: I am parsing arXiv atom entries, which include an affiliation element like this:

<author>
   <name>G. G. Kacprzak</name>
   <arxiv:affiliation xmlns:arxiv="http://arxiv.org/schemas/atom">NMSU</arxiv:affiliation>
</author>