[qtcontacts-sqlite] Improve support for changelog / timestamps

This commit adds support for automatic timestamp generation.

If no creation timestamp exists at creation time, it will be
automatically generated (either the current datetime, or the
modification timestamp if that exists).

If no modification timestamp exists at modify time, it will be
automatically generated (as the current datetime).  Note that
if you want this timestamp to be consistently updated over time
across multiple saves, you need to _manually reset_ the value
between each save operation.  This is because it assumes that the
"true" modification timestamp might come from a sync source.
Merge pull request #52 from chriadam/changelog

[qtcontacts-sqlite] Improve support for changelog / timestamps