Converted current meta information parsing in DocBook to a more extensible version which is aware of the more recent meta representation.
Added recognition of authorgroup element and releaseinfo element to DocBook reader.

Closes #1214
DocBook reader:  set "author" not "authors".
Removed trailing whitespace.
DocBook reader:  set metadata "author" not "authors"
DocBook reader/writer tests:  use "authorgroup".
DocBook template:  use authorgroup if there are authors.