Google Compute Engine Provider

This adds a Google Compute provider to Terraform, which can managed the following resources:

* `google_compute_address`
* `google_compute_disk`
* `google_compute_firewall`
* `google_compute_instance`
* `google_compute_network`
* `google_compute_route`

There are some missing features such as adding/removing disks but those will be added shortly. It is perfectly workable now.