adding Google Compute Engine modules and inventory plugin

Contributing Google Compute Engine modules and inventory plugin.  These modules and plugin require the libcloud library, version 0.13.3 or later (note that this version has not yet been released, but is available in 'trunk' branch of https://github.com/apache/libcloud).

The following modules are included:
* `library/cloud/gce` - create/destroy GCE instances (virtual machines)
* `library/cloud/gce_lb` - create/destroy GCE load-balancer resources
* `library/cloud/gce_pd` - create/destroy unformatted GCE persistent disks
* `library/cloud/gce_net` - create/destroy GCE networks and firewall rules
* `plugins/inventory/gce.{py,ini}` - supports both `--list` and `--host` options
* `test/gce_tests.py` - custom functional test script for all GCE modules

Detailed instructions for setting up your environment can be found in the comments of `ansible/test/gce_tests.py`.  High-level overview is that you must have:
* A valid Google Cloud Platform account with Google Compute Engine and billing enabled.
* A Service Account with valid client email address and private key file (converted to PEM format).
* A supported version of libcloud installed and configured with your GCE credentials.