[SPARK-2024] Add saveAsSequenceFile to PySpark

JIRA issue: https://issues.apache.org/jira/browse/SPARK-2024

This PR is a followup to #455 and adds capabilities for saving PySpark RDDs using SequenceFile and Hadoop OutputFormats.

* I mainly used Python doctests as unit tests. I think it complements #455 by showing some code examples of using the APIs, including ```saveAsHadoopDataset/hadoopRDD```, ```saveAsHadoopFile/hadoopFile```, ```saveAsSequenceFile/sequenceFile```, both old and new MapReduce APIs, ArrayWritable and MapWritable examples.

* Added the missing test case on reading SequenceFile with ```ArrayWritables``` in tests.py

cc @MLnick @mateiz @ahirreddy @pwendell
This line looks longer than 100 characters, change the indentation a bit to prevent that