Add ability to 'save' dives

Add ability to 'save' dives

This just generates another xml file.  Don't get me wrong: I still don't
like xml, but this way we can save in the same format we load things
from.  Except the save-format is a *lot* cleaner than the abortion that
is Suunto or libdivecomputer xml.

Don't bother with some crazy xml library crap for saving. Just do it!

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