Upgraded to work with cozy-ical >= 1.0 and multi-calendar support

We should wait for https://github.com/cozy/cozy-ical/pull/12 before merging this since it has been tested with the cozy-ical version in the PR.

I also added multi-calendar support.
In doctypes Event and Alarm, the first tag corresponds to the element's calendar, other tags are just tags. We might change this but that's code from the Calendar app so I was sure it would work.