Description of problem:
PressGang can aggregate the authors of various topics into an authorgroup for a book, but cannot specify an <editor> or <othercredit> for a book. <othercredit> can take various roles; existing documentation uses "other", together with the <contrib> element. We need "translator" as well, but I think that requires a separate mechanism (and therefore separate bug)

Version-Release number of selected component (if applicable):
Build 201309081012

How reproducible:
100%

Steps to Reproduce:
1. Try to specify the <editor> or translator of a <book>

Actual results:
Don't find a way

Expected results:
Be able to specify the editor of a book or other people who made specific contributions to it.

Additional info:
It might make sense to enable these both at a wholistic, book level via the topic map, and on a topic-by-topic basis. 

Consider also the case where a book might contain work that has come from outside the normal authorship group and might need to be credited separately; for example, the engineers whose work was subsumed into the appendixes of the Red Hat Enterprise Linux Installation Guide: https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Installation_Guide/index.html