[wip] Switch to iterator-based list

Switched Plan as an example. Feedback before I move the pattern to all?
I'm going to get rid of the *List structs and rename ListResponse to
ListMeta.

r? @zenazn @metcalf 
it technically doesn't, but it does change with each page, so that's kind of weird since it's pretty hard to know when to look for it. OTOH I couldn't really come up with good scenarios when one would care about the data there (URL, has_more, total_count -- which isn't populated unless you pass an additional filter)