Requesting an Assertion by ID

In current core the only way to specify an AssertionD in a request is by
including a Subject element with an AssertionSpecifier (which can include an
Assertion or an AssertionID.) The semantics of doing this are not specified,
but elsewhere, the use of Assertion Specifier in Subject is taken to refer
to the Subject of the referenced or included Assertion. 

In other words, the request would be interpreted as asking for Assertions
containing the same Subject, rather than asking for an Assertion with a
specific AssertionID.

I think we need to be able to ask for assertions by ID, so this needs to be
added or the non-intuitive semantics of SubjectSpecifier in Requests
documented.

Hal