[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
We can't write arrays in Scala either (the implicit conversion from Array to ArrayWritable is marked private). Otherwise, it can be awkward as we can't read it back since ArrayWritable doesn't have a no-arg constructor. For user supplied ArrayWritable subtypes, we can read them, it's just they won't be implicitly converted. Essentially the same support as we have in Python.