Add 'datetime' parsing for libdivecomputer xml files

I think this gets me dates on all my dives.  So now I could start
sorting them and removing duplicates.

But before I try to remove dups, I guess I should compare the
libdivecomputer ones against the suunto ones.  Because I bet they have
various "interesting" issues like using Bar vs Atm etc.

"But XML is portable". Crazy people.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>