Do not use Reader::hrefToUid to determine the UID of an incidence

For some servers (e.g. Owncloud), the uid of an incidence cannot be
derived from its uri. As a consequence, remote incidences are not
correctly mapped to local incidences when Reader::hrefToUid is used.
Merge pull request #34 from sdrave/remove_hrefToUid

Do not use Reader::hrefToUid to determine the UID of an incidence