[#5] MemberDataSheet: Add generating a member data sheet

Elwig/Documents/Document.cs

@@ -67,6 +67,8 @@ namespace Elwig.Documents {
                 name = "Letterhead";
             } else if (this is DeliveryConfirmation) {
                 name = "DeliveryConfirmation";
+            } else if (this is MemberDataSheet) {
+                name = "MemberDataSheet";
             } else {
                 throw new InvalidOperationException("Invalid document object");
             }