Pagination function that returns one item at a time

Moved __page_result into a second generator function,
so that users have the choice to apply both.

get_stats_by_adaccount should not assume we want particular
kind of pageview, but I left it for backwards compatibility.
Adding docstrings
Adding docstrings for pagination-handling generator functions
Reindent line causing iterators to spin forever
Merge pull request #44 from GallopLabs/page-results

Generators that handle facebook pagination