Merge pull request #88 from rxin/clean

Made the following traits/interfaces/classes non-public:

Made the following traits/interfaces/classes non-public:
SparkHadoopWriter
SparkHadoopMapRedUtil
SparkHadoopMapReduceUtil
SparkHadoopUtil
PythonAccumulatorParam
BlockManagerSlaveActor
(cherry picked from commit aa61bfd39962a3e3c8ba8e4bcf25f48212933cce)

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