Add azure provider for blobstore_client.

This commit adds the Azure Block Blob provider to the blobstore_client library. It comes unit and functional tests. The functional tests were executed on the real Azure service, so I'm not sure how it will behave with the Blobstore Emulator (after you force it to listen to all interfaces).

We also used it successfully in our tests to create release blobs (`bosh add blob`) and final releases with the bosh_cli (`bosh create release --final`).