Floating times reports

New PR, this time targetting the 2.1 branch.

See #548

Need to implement the following features:

* We need to respect the CALDAV:timezone element in calendar-query and calendar-multiget report when doing expansions and time ranges.
* Fall back on CALDAV:calendar-timezone property if this was not specified.
* Respect CALDAV:calendar-timezone property when doing freebusy queries.