Add --buildmetadata to records AMI info

I'm using ec2debian-build-ami to build images via Jenkins.
I would like to use the metadata from the image creation process
to automate other phases of deployment.

--buildmetada adds a shell script file output option containing
variables which can be sourced into other scripts so you can
use the values in an scripted fashion.
Add a filename parameter placeholder
Respect white space don't use tabs.
Add build metadata using a plugin
Use recommended global checking idiom
Merge pull request #63 from google/master

Rename script + add support for Google Compute Engine
Merge pull request #60 from yanfalies/master

Add a build metadata output file to record the ami and snapshot ids for further scripting
Readd some task fixpoints that got disappeared.
Amend README to include GCE
Fix #57
Fix #46. The fix for #40 simply introduces a new problem.
Add plugin by yanfalies to the list of plugins
Fix #64. Do not delete the apt-cache.
As Charles noted, it only serves to annoy users when trying
to install things on a fresh instance.
Fix links and commands in documentation after renaming the project
Update reference to the renamed "debian-cloud-chef" plugin
Merge pull request #65 from tmatilai/renames-in-documentation

Update references to renamed projects
Vary APT mirror per cloud + make it configurable

Google is now running a Debian mirror inside Google Compute Engine so
that images within our cloud can save on bandwidth costs. Use it by
default for our images, without changing Amazon EC2.

Also make the mirror configurable with a new --apt-mirror option.