Small fix for Python 2.5

.gitignore

@@ -7,3 +7,5 @@ build/
 dist/
 mongoengine.egg-info/
 env/
+.project
+.pydevproject

mongoengine/base.py

@@ -491,6 +491,7 @@ class BaseDocument(object):
 
 if sys.version_info < (2, 5):
     # Prior to Python 2.5, Exception was an old-style class
+    import types
     def subclass_exception(name, parents, unused):
         return types.ClassType(name, parents, {})
 else: