NG: Microsoft Azure storage driver

This is the storage driver implementation to enable hosting the storage backend of the next-generation docker-registry on [Microsoft Azure Blob Storage Service](http://azure.microsoft.com/en-us/services/storage/). In order to fully run the tests, an Azure storage account will be needed. In case you need one, please contact me or you can [sign up for Azure](http://azure.microsoft.com/en-us/pricing/free-trial/) if you don't have an Azure account and create a storage account yourself.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

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/cc: @stevvooe @dmp42 @johngossman
If the transfer is interrupted, nothing gets written to the cloud storage until the HTTP request is completed. Azure Storage API is transactional, therefore there's no "partially written or corrupt block" like a local filesystem. If the registry process dies and re-sends the same chunk at the same offset, request will be accepted if the initial upload request was interrupted in the middle. In fact this sounds like a good candidate for storagedriver/testsuite.