django_on_call.models: Add an OnCall class
django_on_call.models: Add an OnCall class

I'm having second thoughts about the exec() approach, but parsing
recurrence rules seems too complicated for now.  I may add something
inspired by RFC 5545 later [1].

[1]: http://tools.ietf.org/html/rfc5545#section-3.3.10

wking/django-on-call