Increase delight.

Add convenience method SignedXml::Document which forwards its
argument to Document.new.

Document.new can now take any argument Nokogiri::XML.parse can. If
the argument is already a Nokogiri::XML::Document, that object is
used directly; otherwise, the argument is passed to Nokogiri::XML.
Add Fingerprinting utility module and use it.
Whoops. Add the actual new module.