Correctly place floating events in freebusy

If a collection has an associated timezone, this change means it is used for determining where a VALUE=DATE or TZIDless event should be placed in freebusy.

If the collection has no timezone, it is assumed UTC, as before.

This screenshot shows the new behaviour for a VALUE=DATE event (in thunderbird's freebusy display) -- the all-day event in a zoned calendar is now aligned to the day correctly, where previously both would be incorrect:

Resolves #146 (closed) and #169 (closed).

This change broke several tests, which I have fixed - I believe the test data was encoding the old "assume-UTC" behaviour.