[buteo-sync-pluging caldav] Own Owncloud improvements.

Following commits missed proper changelog format:
- Do not use Reader::hrefToUid to determine the UID of an incidence.
- Use correct URI for DELETE requests.
- Fix ICal data encoding to match the Content-Type header.
- Let Reader class parse all retrieved iCalData.
- Refactor Report class, move processing of ETags to NotebookSyncAgent.