SPARQL cryptographic hash algorithms

For ACTION-536 I asked W3C's security lead, Thomas Roessler, which hash
algorithms we should use and he agreed with:
XML Signature Syntax and Processing Version 2.0
http://www.w3.org/TR/xmldsig-core2/#sec-MessageDigests

which, in the current WD says SHA-1 (for backward compatibility),
SHA-256, SHA-384, and SHA-512.

Including all four is one or two more than the commenter wants, but
seems warranted in this case, I think.

   -- Sandro