Remove delete_notebook method from Contents

This method is no longer part of the API.
Adds gapi_utils.gapi_ready guard to Contents.delete

gapi_utils.gapi_ready is a promise that is fullfilled when the Google API has loaded.  This promise should be used to guard any calls to gapi_util or drive_util functions, to prevent calls to the Google API before the API has loaded.
Merge pull request #13 from KesterTong/fixes

Fixes