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.
don't use async.parrallel. Cozy evolves in low resource environment, series execution fits better with it.