Moved the safe handles for credentials and contexts into their own file.

Credentials/SafeCredentialHandle.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace NSspi.Credentials
+{
+
+    public class SafeCredentialHandle : SafeSspiHandle
+    {
+        public SafeCredentialHandle()
+            : base()
+        { }
+
+        protected override bool ReleaseHandle()
+        {
+            SecurityStatus status = CredentialNativeMethods.FreeCredentialsHandle(
+                ref base.rawHandle
+            );
+
+            base.ReleaseHandle();
+
+            return status == SecurityStatus.OK;
+        }
+    }
+
+}