Azure Storage Driver config and README updates

This is our [Docker Global Hack Day #2][hackday] project!

We were able to run the docker registry on [Azure Blob Storage](http://azure.microsoft.com/en-us/documentation/services/storage/) using our [**`docker-registry-driver-azure`**](https://github.com/ahmetalpbalkan/docker-registry-driver-azure).

`docker-registry-driver-azure` is not yet ready via `pip install` but we provide a docker image called [ahmetalpbalkan/registry-azure](https://hub.docker.com/u/ahmetalpbalkan/registry-azure/) on Docker Hub. But we're planning to make it available via pip as well, soon.

We called this new flavor name "**azureblob**" instead of "azure" because then inside azure.py, we cannot import [Azure Python SDK](https://github.com/Azure/azure-sdk-for-python) using _"import azure"_ or _"from azure import storage"_

[hackday]: https://blog.docker.com/2014/10/announcing-docker-global-hack-day-2/