Digest stuff - issue 25

Request
	- uses signature block

Response
	- add in <RequestSignature> element


RequestSignature carries the authentication of the request in the
response, providing a cryptographic linkage between the request and the
response.

This means that the client is only dependent on the service ability to
validate the request signature and not dependent on the service
correctly applying the authentication procedure.

Service MUST NOT include request unless request signature was validated
successfully.

Service should support SHA1 mechanism

ProtocolMechanism identifier is RequestSignature