proposed terminology addition to STP - impersonation

Donn Flinn asked that a definition of impersonation be added to the
terminology section of the SAML token profile.

I propose that the following line be added (at about line 183).

Impersonation – occurs when the attesting entity is not the subject of 
the assertions.

where attesting entity is already defined as:

175 Attesting Entity – the entity that provides the confirmation 
evidence that will be
used to establish the correspondence between the subject of SAML subject
statements (in SAML assertions) and SOAP message content.