[SPARK-1690] Tolerating empty elements when saving Python RDD to text files

Tolerate empty strings in PythonRDD

Author: Kan Zhang <kzhang@apache.org>

Closes #644 from kanzhang/SPARK-1690 and squashes the following commits:

c62ad33 [Kan Zhang] Adding Python doctest
473ec4b [Kan Zhang] [SPARK-1690] Tolerating empty elements when saving Python RDD to text files
(cherry picked from commit 6c2691d0a0ed46a8b8093e05a4708706cf187168)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>