snapshot_blob() needs to return a value that we use to pass in for the snapshot parameter to get_blob().

From the documentation at
http://msdn.microsoft.com/en-us/library/windowsazure/ee691971

x-ms-snapshot:

This header returns a DateTime value that uniquely identifies the snapshot. The value of this header indicates the snapshot version, and may be used in subsequent requests to access the snapshot. Note that this value is opaque.

I have added a test, currently commented out, that demonstrate how this may work: test_get_blob_with_snapshot