Use the OAuth2 decorator's http object with the discovery API

This is a minimal change to demonstrate using the http object from the OAuth2 decorator instead of constructing an httplib2.Http object manually. This is required because the manually constructed http object isn't authenticated.

I didn't test adding a volume to the user's bookshelf from AppEngine. It may still not work due to the IP Address issues you mentioned previously. It's possible that it will work *if* the API looks at the user's country code (in their account) instead. 

hope this helps.