Feature: google compute engine

Adds jclouds-google-compute-engine dependency, and tests.

Note that `ActiveMQGoogleComputeLiveTest` fails strangely: the launch command executes without error but there is no process running. This is not the same issues as https://github.com/brooklyncentral/brooklyn/issues/162. We tried executing the launch command `./bin/activemq start; sleep 10s; ps -ef; cat data/*; true` but this also showed that the process was not running. I confirmed that the ActiveMQ EC2 centos live test is working.

How do you feel about merging this anyway, given the jboss AS7 live test works on GCE (and we'll then be able to use it for brooklyn-cdh etc as well). A subsequent task would be to figure out why ActiveMQ is failing on GCE...