Work on #19. Initial tailer implementation.

TODO: Simplified callback API.
Fixes #19. Implementation, docs, unit test.
Merge branch 'issue-#19-file-tail'
Fixes #20. Option to keep old version of file.
Merge branch 'issues-19-20-etc'
Fixes #22. VDate and VTimeOfDay formatting.
Fixes #21. Improved VSettings XML format control.
Merge branch 'issues-19-20-etc'
Fixes #21. Typo in date format string.