Distribute the iCal parser as a separate module

It would be nice to be able to distribute the SchoolBell iCal parser as a module
independent of Zope and SchoolBell. (discussed a little on IRC)

From the package point of view, a python-icalendar binary package that does not
depend on SchoolBell or Zope and installs the icalendar parser in the standard
python path. (as schoolbell.calendar.icalendar or schoolbell.icalendar)

(I am not sure whether future schoolbell packages will be installed on the
python path or in a zope subdirectory, the current Zope policy seems only valid
for zope 2)

I think there are 3 blockers:

* schoolbell.calendar.icalendar imports from schoolbell.calendar.simple
* difficulties with two schoolbell modules being on the python path.
  (IIRC there is some evil black magic to solve this)
* Some kind of distutils setup that can install the modules separately.
  (The same hack used for schooltool/schoolbell is probably not a good idea)