Support DELETE on calendar.ics

Tom Hoffman reported this (see schooltool-dev@, subject "New Trunk Tracebacks"):

> While testing the new trunk (on my Mac) I got these tracebacks in the
> terminal window, although the server seemed to be working properly:

> raystown:~/Desktop/schooltool hoffman$ python schooltool-server.py
> Reading configuration from schooltool.conf.in
> 2005-09-20 14:41:54,119 zope.server.http (HTTP) started.
> Hostname: raystown.local
> Port: 7080
> Startup time: 23.735 sec real, 7.780 sec CPU
> 2005-09-20 14:43:49,901 http://localhost:7080/persons/manager/calendar.ics
...
> File "/Users/hoffman/Desktop/schooltool/Zope3/src/zope/app/http/delete.py",
> line 37, in DELETE
> dir = IWriteDirectory(container)
> File "/Users/hoffman/Desktop/schooltool/Zope3/src/zope/interface/interface.py",
> line 704, in __call__
> raise TypeError("Could not adapt", obj, self)
> TypeError: ('Could not adapt', <schooltool.person.person.Person object
> at 0x35920f0>, <InterfaceClass
> zope.app.filerepresentation.interfaces.IWriteDirectory>)

My hyphothesis:

> Your iCalendar client tries to perform an HTTP DELETE request on the
> .ics URL before PUTing a new version, I assume. SchoolTool doesn't
> expect the DELETE. It is a minor bug in SchoolTool.

Things to do:

  - Make DELETE on a calendar clear the calendar.
  - Both via REST and the regular HTTP interface.
  - Don't forget ftests first!