Improved log rotation:
  * Implement reload action in init script.
  * Update logrotate script to reload (not restart) and include option
    "notifempty".

Improved testsuite:
  * Make use of python-caldav.
    See <https://github.com/Kozea/Radicale/issues/232>
    and <https://github.com/Kozea/Radicale/issues/330#issuecomment-270259730>
  * check testsuite at build-time (not only in autopkgtest)
  * fix and enable caldav-tester autopkgtest
