caldav: [server] handle PROPPATCH for calendars

[barkyq]

it seems this would necessitate a change in the exported backend interface; perhaps:

PropPatch(ctx context.Context, path string, cal *ical.Calendar) error

One could theoretically reuse CreateCalendar instead of adding new PropPatch, but this seems bad form and might break existing implementations.

[barkyq]

related #169

[barkyq]

Edit: DELETE is already handled (DeleteCalendarObject can also be used to delete Calendar collections)