Using unittest2 in place of unittest

Used for Python 2.6 compatibility in testing.
diff --git a/tests/test_errors.py b/tests/test_errors.py
index 0cef892..9af8490 100644
--- a/tests/test_errors.py
+++ b/tests/test_errors.py
@@ -21,7 +21,7 @@
 __author__ = 'afshar@google.com (Ali Afshar)'
 
 
-import unittest
+import unittest2 as unittest
 import httplib2