Allow us to iterate through the entire list.

Pagination is, somewhat, unique, with Instagram. Whilst you *could* try
to figure out which API takes max_id, max_tag_id, max_like_id, etc. and
follow the changes they make.

It makes sense to, instead, just use the 'next' URL they respond with.

That is not very Pythonic though. But the in-built generator return
result *is*. However it assumes that you only want 3 pages of results.

Make the max_pages parameter optional, and thus we can regularise paging
through the API when using Python.
return the next url for requests where objectify_response=False. Fixes as_generator when requesting endpoints where the response is paginated but not objectified.
Merge git://github.com/aventurella/python-instagram
Merge branch 'fixup-generation' of git://github.com/akumria/python-instagram