More additions based on some discussion with Gunnar in IRC:

More additions based on some discussion with Gunnar in IRC:
 - Allow specifying if a resource is required or optional
 - Add a response_type property to resources. Specifies if REQUESTS are
   either always accepted, always denied, automatically accepted/denied
   based on actual availability, handled manually (still todo), or simply
   ignored (set response to NONE).

Also, try to clean up the implementation - trying to make it more likely
to be able to be extened for kolab. No idea if this helps of not though ;)