Moved the credential files into their own folder.

Credentials/ServerCredential.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace NSspi
+{
+    public class ServerCredential : Credential
+    {
+        public ServerCredential( SecurityPackage package ) : base( package, CredentialType.Server ) { }
+    }
+}