Issue CORE-114: Schema Extension Points

Are current extension points ("any" elements/attributes) sufficient to
support anticipated new features?  For example, to support payload
packetization, we will need to specify "part x of y" parameters.  In
this case, we could choose to utilize PartProperties to declare such
parameters instead of using a schema extension, unless we prefer the
stronger content model that schema could provide.  But there may be
other cases where a schema extension would be invaluable.

In any case, there are no extension points anywhere within
UserMessage, and I want to be sure that whether we add any or not, it
is through a conscious decision.

(The top-level Messaging element has both element and attribute
extension points.  SignalMessage has an extension element, so that we
can add new signal types if necessary.)

--Pete
Pete Wenzel