Fix unittest

[reverbc]

This PR is addressed to the issues describe in #46

For test_python_keyword case: some of the keywords are Python 3 only (e.g. None) and will cause failure in Py26 and Py27. In this case I've added a few more test cases and use six to distinguish them.

For test_invalid_feature and test_invalid_external_dtd cases: using assertRaises as context manager is a feature in Py27 so not compatible with Py26. Switching back to old-school syntax here for backward compatibility.

[coveralls]

Coverage remained the same at 35.514% when pulling 2f78297 on reverbc:fix-unittest into a18c171 on stchris:master.

[stchris]

Closing this after #82