Change .list() to an iterator (instead of returning a list)

Also: make pagination slightly less weird, and fix the sample grab_oncall.py script
I did it with incidents once.

-- 
James Brown,
currently mobile

> On Jun 26, 2014, at 10:03 PM, "Gary M. Josack" <notifications@github.com> wrote:
> 
> In pygerduty/__init__.py:
> 
> > @@ -133,14 +134,17 @@ def list(self, **kwargs):
> >                  this_paginated_result = self._list_no_pagination(**these_kwargs)
> >                  if not this_paginated_result:
> >                      break
> > -                total_result.extend(this_paginated_result)
> > +                for item in this_paginated_result:
> > +                    if item.id in seen_items:
> I haven't come across a case where the API returned the same ID multiple times. Is this common?
> 
> —
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