add support for extracting relations between emails, openids, and photos from...

This adds support for extracting relations between emails, openids, and photos from libravatar's export data.

But support to export script needs to be added too. I only did the import path because I don't have the old libravatar running on my machine right now.

It assumes the following data are generated by the export script:

<?xml version="1.0" encoding="UTF-8"?>
<user xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="https://www.libravatar.org/schemas/export/0.2 https://www.libravatar.org/schemas/export/0.2/export.xsd"
      xmlns="https://www.libravatar.org/schemas/export/0.2">
  <account username="browserid376" site="https://www.libravatar.org"/>
  <emails>
    <email photo_id="34">somenick@gmail.com</email>
    <email photo_id="34">somenick@fedoraproject.org</email>
    <email photo_id="34">somenick@redhat.com</email>
  </emails>
  <openids>
    <openid photo_id="34">http://somenick.id.fedoraproject.org/</openid>
  </openids>
  <photos>
    <photo encoding="base64" format="png" id="34">
    </photo>
  </photos>
</user>

I.e. id attribute for <photo> and photo_id for <email> and <openid> are added here.