SupportCal Dav Free Busy Report

The CalDAV free-busy-query variant of the REPORT allows for a text/calendar object to be returned which is simply a set of times which are busy, and a set which are tentatively busy, containing no information.

This was added for release 0.6.0 and the code used in constructing this was also applied back to the /freebusy.php code in order to make this a more robust freebusy solution, since it previously did not deal correctly with iCalendar repeat rules - a rather critical part of this whole process!