Docx writer: Implement track changes.

These have default authors and dates of "unknown" and timestamp-zero,
respectively.
Docx writer: Default to user login and time of change if not given.
Docx writer: Accomodate GHC 7.4 (no lookupEnv)
Merge pull request #1564 from jkr/trackChangesWriter

Docx writer: write track changes.