Low-level: Implement DCE Extensions

The GSSAPI DCE Extensions include support for wrapping and unwrapping
IOV and AEAD messages (this allows for DCE RPC and other SSPI
functionality).

Part of #6

Author: DirectXMan12

gssapi/raw/__init__.py

@@ -43,3 +43,9 @@
     from gssapi.raw.ext_password_add import *  # noqa
 except ImportError:
     pass
+
+# optional DCE (IOV/AEAD) support
+try:
+    from gssapi.raw.ext_dce import *  # noqa
+except ImportError:
+    pass