Check for new and updated non-public calendars

* Add new calendars to the list as they show up.
* Reload non-public calendars if they are updated on the server.

Fixes #31.

This works but it is not an optimal solution:
* If a calendar changes it reloads the current view
  no matter wether anyting in the view changed or not.
* If a calendar changes it drops all cached events
  even if they did not change.
* If an event is added to the calendar or moved
  this calendar will also be reloaded because the new event
  changed the syncToken on the server.
* If a calendar changed
  it will be added to the bottom of the list of calendars.

I would be fine with most of these but the last one needs to be fixed.

Signed-off-by: Azul <azul@riseup.net>