[orage] Adding events to external files

[Florent Teichteil]

> Using latest GIT of Orage, I happily do my syncing of Orage's event
> file using SpiderOak, across several machines. Works very nicely
> sofar.

So, we should perhaps create a general wiki page about syncing orage's
events across several machines, where we would have subsections for
SpiderOak, gcaldaemon for google calendars, or any other solution. What
do you think?