[XML Signature] Changes to Core 19

Hi,

	As we didn't get time to discuss stuff today, I am iteratively addressing
the various issues. As some of them would be cascading, let us work our way
thru them - bottom-up.

	Tier 0: I formally propose the following changes to the Schema:


	1.	Add XML Signature to AssertionAbstractType:
		247. 1 <element ref = "ds:Signature" minOccurs="0" maxOccurs="1"/>
		1189. 1 <element ref = "ds:Signature" minOccurs="0" maxOccurs="1"/>

	2.	Add XML Signature to RequestAbstractType:
		1421. 1 <element ref = "ds:Signature" minOccurs="0" maxOccurs="1"/>

	3.	Add XML Signature to ResponseAbstractType:
		1492. 1 <element ref = "ds:Signature" minOccurs="0" maxOccurs="1"/>

	4.	Collapse SingleAssertionType and MultipleAssertionType to one structure.

		Reasons : Easy to understand and comprehend. Makes way for multiple
assertion signing issues later.
			IMHO, the MultipleAssertionType with minOccurs="0" does not make any
sense. From what I understand, the current MultipleAssertionType makes sense
only with minOccurs="2" as with minOccurs=1 it is a SingleAssertionType.

		Proposal :

		Delete 1197 thru 1224.

		Add @1187.1 : 	<choice minOccurs="1" maxOccurs="unbounded">
					 	<element ref="saml:Statement"/>
						<element ref="saml:SubjectStatement"/>
						<element ref="saml:AuthenticationStatement"/>
						<element ref="saml:AuthorizationStatement"/>
						<element ref="saml:AttributeStatement"/>
 					</choice>

cheers