[SPARK-2948] [SPARK-2910] [SPARK-2101] Python 2.6 fixes

- Modify python/run-tests to test with Python 2.6, if available.
- Use unittest2 when running on Python 2.6.
- Fix issue with namedtuple.
- Skip TestOutputFormat.test_newhadoop on Python 2.6 until SPARK-2951 is fixed.

Closes #1868.  Closes #1042.
[SPARK-2954] Fix MLlib _deserialize_double on Python 2.6.