[PySpark] [SPARK-2954] [SPARK-2948] [SPARK-2910] [SPARK-2101] Python 2.6 Fixes

- Modify python/run-tests to test with Python 2.6
- 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.
- Fix MLlib _deserialize_double on Python 2.6.

Closes #1868.  Closes #1042.

Author: Josh Rosen <joshrosen@apache.org>

Closes #1874 from JoshRosen/python2.6 and squashes the following commits:

983d259 [Josh Rosen] [SPARK-2954] Fix MLlib _deserialize_double on Python 2.6.
5d18fd7 [Josh Rosen] [SPARK-2948] [SPARK-2910] [SPARK-2101] Python 2.6 fixes

Conflicts:
	python/pyspark/mllib/_common.py
	python/pyspark/serializers.py
	python/pyspark/tests.py