RFC6330 conformity

We were doing really bad stuff.
* we were not using the correct block size, but instead
  creating something completely out-of standard
  This surely means that we were not compatible with
  OpenRQ (Ticket #20), and possibly a reason for which
  decoding fails too ofter (Ticket #21).
* We used the wrong ESI number, which should skip
  the padding numbers (#23, #20

)
* we did not expose the real block size to the user
  of the RFC namespace (RFC iterators)

Signed-off-by: Luca Fulchir <luker@fenrirproject.org>