Add ability to get/list snapshots in GCE.

This patch...
1. Adds the Snapshot collection and model
2. Adds the get/list requests
3. adds some examples
Merge pull request #2065 from maginatics/gce_get_list_snapshots

[google|compute] get/list snapshots
[openstack|compute] update volume tests
Merge pull request #2066 from burns/openstack_volume_tests

[openstack|compute] update volume tests