exif: use DateTimeOriginal if present. Added test for it.

Change-Id: Idfd75b15d7302b5c3a6fbe3464917b16bce56558
sigdebug: update disco global variable to be Camli.config. re-run make embeds

Change-Id: If03e8d2fbd709d90810bedac8506058a4026dc04
server.vivify: use the right claim date on all 3 relevant dates.

Fixes https://code.google.com/p/camlistore/issues/detail?id=93

Change-Id: I9d9ab02d47f2ef78f6f690414a04fc5c24f95fac