Enabling easy addition of new data stores, adding Windows Azure Blob Service support

We've worked on extending WAL-E to Windows Azure Blob Service as a backup storage option w/o effecting current S3 functionality.  The blob store used by WAL-E on a given run is set from the scheme for the given storage prefix, i.e. `s3://<bucket name>/<path…>` for S3 storage and `wags://<container name>/<path…>` for Windows Azure Blob Storage. Additionally, the methodology used for this update is intended to simplify WAL-E's expansion to other platforms as needed. Adding a third data store can now be done trivially.