Improvement/revamp cursor iteration
Address issue #41, #42 and #43.
I tried to work around the current design in master to fix all corner cases but was unable to do that and, at the same time, actually taking into account the skip/limit at the url level.
Most 'limited' fixes (changing as little code as possible) would have required some sort of duplication of code between cursor.__iter__ to cursor.iter_pages and shared responsibilities of the skip/limit logic. I felt that it was cleaner to centralize that logic in one place and that this place had to be the one closer to the api calls.
Therefore, the skip/limit logic has been moved from cursor.__iter__ to cursor.iter_pages so that query parameters used to query AppNexus are actually impacted by the user's configuration.