Algorithm URI corrections

Sigh, sorry, I didn't catch this, but there are too many specs around
defining the algorithms.
 
The correct algorithm URI for RSA w/ SHA256 is:
http://www.w3.org/2001/04/xmldsig-more#rsa-sha256

The reference is:
[RFC4051] D. Eastlake 3rd. Additional XML Security Uniform Resource
Identifiers. RFC 4051 April 2005. http://www.ietf.org/rfc/rfc4051.txt

We also probably should add the explicit reference to SHA-256 as a digest,
which comes from XML Encryption: http://www.w3.org/2001/04/xmlenc#sha256

The reference is:
[XMLEnc] D. Eastlake et al. XML Encryption Syntax and Processing. World Wide
Web Consortium. http://www.w3.org/TR/2002/REC-xmlenc-core-20021210/

-- Scott