[caldav] Ensure uris and etags are up to date

Set the 'uri' custom property of locally inserted incidences. Moreover,
check for all locally inserted or modified incidendes if an etag has
been received for the PUT request. If not, the incidence is reloaded
from the server.
Merge pull request #36 from sdrave/update_etags

Ensure uris and etags are up to date