Merge pull request #99 from google/master

Commit corresponding to Google 20130816 images
Handle GPG key for Compute Engine apt repository

Google's repository is now GPG-signed, so builds for Google Compute Engine have
to handle the key for apt not to fail on the unknown key.
Compute Engine: prepare for Debian kernel support

Google Compute Engine has not yet launched the support required for the Debian
kernel, but these changes are in preparation for when that does occur. Until
then, our images will continue to work with the Google-provided kernels.