Gcloud

Actually two commits :

- move to extensions over conventions
http://forums.gradle.org/gradle/topics/is_the_new_plugin_extension_approach_the_way_to_go

- add in gcloud functionality
I recommend having the "preview", "app", "deploy" bit go in the GCloudCommandBuilder constructor. Those arguments are fundamentally different than the two positionals that you .add() later. They're identifying the command rather than its arguments.