Added "next_page" result parallel to "models" and "cursor" when returning search results.
Updated python client to use "next_page" result from Search calls.
Updated tests to pay attention to next_page results
Python client Query.fetch returns (models, next_page):
models is a list of results
next_page is a query that, when fetched, returns the next set of results.
next_page can be None.

Updated tests to work with new search.
Added comments for python client.
Streamlined Query class to allow multiple calls to fetch() - simplifying process of making several consecutive calls to get additional data.

Updated tests to use streamlined Query class.