Add assert_not_in method for Python2.6

IPython/testing/nose_assert_methods.py

@@ -9,3 +9,9 b' def assert_in(item, collection):'
 
 if not hasattr(nt, 'assert_in'):
     nt.assert_in = assert_in
+
+def assert_not_in(item, collection):
+    assert item not in collection, '%r in %r' % (item, collection)
+
+if not hasattr(nt, 'assert_not_in'):
+    nt.assert_not_in = assert_not_in