Rewriten SimpleSign page

[Nat Sakimura <n-sakimura@nri.co.jp>]

I have re-written the XrdOne/SimpleSign page to reflect the talks today.

http://wiki.oasis-open.org/xri/XrdOne/SimpleSign

Hope this is simple enough and close to an agreement.

I have changed <SXRD> element name to <SDSIG> to generalize it.
Perhaps that's too much of a generalization though.
Also, instead of ds:KeyInfo, I have flattened things and defined
<CertURI>, <Certs>, <SigAlg>.

We do not gain much by borrowing just ds:KeyInfo from XML Dsig.
If we have XML Dsig processor, we would be using XML Dsig after all.

=nat