code: function newMethod(name, accessor) {
          var impl = OAuth.SignatureMethod.REGISTERED[name];
          if (impl != null) {
              var method = new impl();
              method.initialize(name, accessor);
              return method;
          }
          var err = new Error("signature_method_rejected");
          var acceptable = "";
          for (var r in OAuth.SignatureMethod.REGISTERED) {
              if (acceptable != "") acceptable += '&';
              acceptable += OAuth.percentEncode(r);
          }
          err.oauth_acceptable_signature_methods = acceptable;
          throw err;
      }
docs: Instantiate a SignatureMethod for the given method name.