Add starting_token and max_items to paginators

When calling .paginate() on an operation you can specify two
additional parameters:

* starting_token - Indicates where to start in the pagination
* max_items - Indicates the total number of results to return.
  The results are based on the primary result key (the first item
  if there are more than one result key).
Fix pep8 issues

As per review feedback.
Merge branch 'unify-page' into develop

Closes #88.

* unify-page:
  Fix pep8 issues
  Add starting_token and max_items to paginators