Sorts items in Contents.list_contents

Sorts items so directories appear before files, and items are sorted alphabetically within each category.
Use pagination of results from Google API

The Google API sometimes returns results in paginated form, i.e. it returns results N at a time, where there is some upper limit to the N that users can specify.  This commit uses this mechanism properly, so that directories containing more than 1000 items can still be displayed.