Handle end of pagination

iter_pages crashes when attempting to paginate past the final page.
`next_url = json_response['paging']['next']` has a KeyError when the
json response lacks a 'next' key. The 'next' key is not included if this
is the last page of data.

https://developers.facebook.com/docs/reference/api/pagination/
Merge pull request #21 from mkenyon/fix_iter_pages

Handle end of pagination