Allow affiliation inside author

[tomschr]

Problem

The author doesn't allow information about affiliation. This is needed for SUSE Best Practices.
(Reported by @chabowski).

Solution

@sknorr and I agreed that we need the following content model:

affiliation ::= (jobtitle | orgname | (jobtitle & orgname))

Example

<author>
    <personname>
     <firstname>Tux</firstname>
     <surname>Penguin</surname>
    </personname>
    <email>tux@example.org</email>
    <affiliation>
     <jobtitle>Ice breaker</jobtitle>
     <orgname>Icelandic Corporation Inc.</orgname>
    </affiliation>
</author>