Azure Blob Storage Client Library for Go

This contains set of commits to implement a blob storage client. This is mainly required for Azure Storage Driver for [docker/docker-registry](https://github.com/docker/docker-registry/tree/next-generation)'s new implementation.

Mostly the parts that would be required to the driver are implemented. Therefore some of the parts are not implemented yet. These are:
* Blob/container leases
* Operations on Page Blobs
* Snapshot blob
* Abort blob copy
* Set/Get blob/container metadata
* Set/Get container ACL
* Get/Set blob service properties
* Get Blob Service Stats

On the other hand most of the basic functionality and Block Blob methods are provided within the library.

This package has no dependency to the rest of the repo at all. I could not find any reusable parts. Would be good if we can take this in as soon as possible.

cc: @jeffmendoza