Email binding, notes

Attached please find a sketch of a binding of the one-way MEP to email,
taking the existing email binding note as a starting point.  As one
would hope, the meat of it is in the mapping between properties and
email content (Table 2) and in the mapping of error conditions to SOAP
faults (which I've completely punted -- but then am I required to do
anything?).

Random notes taken along the way:

    * The original refers to the RequestingSOAPNode and
      RespondingSOAPNode roles.  We don't define these.
    * Hmm ... what if a given node gets the sent email twice?  This is a
      general issue.  I don't think we specifically mention it in the
      MEP spec yet.
    * It looks like it might be better to have a single Message
      property, instantiated at each node, rather than separate
      InboundMessage and OutboundMessage properties, with only one or
      the other instantiated at a given node.  Most of Table 2 is
      identical for sender and receiver.  This is the only exception.
    * The original binding treats SOAP nodes as being attached to email
      interfaces.  This seems good and right, and I follow it.  But I
      note that a given email interface need not have a SOAP node
      attached to it, in which case a message sent will normally reach a
      receiver, but not a SOAP node.  For that matter, there could be
      multiple SOAP nodes attached to a given email interface and
      address (or to the same address at different interfaces, depending
      on what we mean by an email interface).  In such cases, a single
      message sent to a single address will still reach multiple SOAP nodes.
    * Goodness, those state tables took up a lot of room.