Add support for Azure Copy Blob

Presently lacks support for CopyBlobOptions.  API reference:

http://msdn.microsoft.com/en-us/library/dd894037.aspx
Unsure about `AddCopySourceHeader`; this feels like a clumsy way to implement a compound header.  I modeled the interface after the S3 equivalent but perhaps this is the wrong approach. If I change `fromContainer` and `fromName` to `fromUrl` I can avoid this entirely and expose the full power of the Azure interface.  Thoughts?