Allow reverting to a timestamp.

Given a timestamp, this finds the version that was current at the desired
time.  For example, if version 1 was made at 10:00, and version 2 at 11:00,
if you revert to 10:30, it will give you version 1.

And, enough with Arel.  It's making things difficult...

Signed-off-by: Chris Riddoch <criddoch@globe.gov>