Fill in more chnages for 2014-05-26 for Snapshots.

* Add AutoSnapshot-related models and methods.
* Update DeleteSnapshot not to require DiskId.
* Update DescribeSnapshots to have all parameters optional and be paginated.
* Update DescribeSnapshots to be able to filter based on DiskId,
* InstanceId, and up to 10 (API-enforced, not in here) SnapshotIds.
* Update the Snapshot model to include SourceDisk information.
Doc updates.