Add support for Google Compute Engine

This commit is the result of collaboration among David McWherter, Anders
Ingemann, and me. The rename to build-debian-cloud is at Anders's
initiative.

David's and my work on this is Copyright 2013 Google, Inc. and is
released under the Apache License 2.0.
Re-add accidentally removed plugins/README.md text
Different volume_size defaults for EC2 and Google

In keeping with Google Compute Engine's other images, default to 10 GB
for our Debian images. Image size is handled differently for Amazon EC2,
so leave the pre-existing 1 GB default alone there.

Also update the comment in the affected build-debian-cloud section to
match reality (the --filesystem option already exists).
Remove /etc/resolv.conf during image creation

The build system's DNS info should not pollute the image. DHCP will
recreate this for the VM environment.

Also renumber tasks/gce/52-* to tasks/gce/25-* so that those steps occur
when DNS is still functional inside the image.
Clean up loopback devices and mounts on all exits

Previously, depending on where a Google Compute Engine image build
failed, the loopback mount could linger, which is cumbersome. This
cleans that up, though it leaves the workspace around during unclean
exits to facilitate debugging.
Adjust two traps to co-exist

Turns out a pre-existing use of 'trap' in Anders' code stepped on the
trap I added. This coordinates, in a way that shouldn't break non-Google
builds.
Merge pull request #63 from google/master

Rename script + add support for Google Compute Engine