add namespace capability to the current BlobBackend interface

and modify the FSBlobBackend accordingly.

the write, read and list methods of the BlobBackend interface should allow a namespace, to be used by the IncomingBox implementation.

I would like to discuss the idea of enforcing a fixed-len namespace preffix that matches cleanly the folder boundary in the current FS implementation (this means that ALL incoming blobs would be written to the same folder, ie, a single one).