Extended libbe.storage.base for separate read/write control.

Extended libbe.storage.base for separate read/write control.

Rather than just having .read_only to set write permissions and
assuming that read was always legal.  We also added user and backend
control of both readable and writeable:
  do you want to read/write?
and
  can you read/write?

Specialized NotSupported into NotWriteable and NotReadable.

Added automatic unicode encoding on .set(), and decode option on
.get().