Add an alternative way to iterate over all pages in an index query.
Add tests for pagination over an entire 2i query.

Also fix bug in PB handling of continuations. Previously we assumed
the continuation would be `None` if not present, but in fact it is the
empty string. Checking HasField('continuation') resolves that issue.
Fix PEP8
Add information about default value for `max_results` when paginating.
Merge pull request #286 from basho/feature/gh286-paginate-indexes

has_next_page for indexpage should return true at least once

Reviewed-by: javajolt