Implemented checkpointing of StreamingContext and DStream graph.
Fixed bugs in the streaming testsuites.
Refactored DStreamSuiteBase to create CheckpointSuite- testsuite for testing checkpointing under different operations.
Minor modifications.
Updated Kryo to 2.20.
Merge tag 'v0.6.0' into python-api
Update Python API for v0.6.0 compatibility.
Added functionality to forget RDDs from DStreams.
Add PySpark README and run scripts.
Fixed some bugs in tests for forgetting RDDs, and made sure that use of manual clock leads to a zeroTime of 0 in the DStreams (more intuitive).
Added MapValueDStream, FlatMappedValuesDStream and CoGroupedDStream, and therefore DStream operations mapValue, flatMapValues, cogroup, and join. Also, added tests for DStream operations filter, glom, mapPartitions, groupByKey, mapValues, flatMapValues, cogroup, and join.
Remove PYTHONPATH from SparkContext's executorEnvs.

It makes more sense to pass it in the dictionary
of environment variables that is used to construct
PythonRDD.
Renamed examples
Modified API of NetworkInputDStreams and got ObjectInputDStream and RawInputDStream working.
Renamed the network input streams.
Updated StateDStream api to use Options instead of nulls.
Renamed APIs to create queueStream and fileStream.
Renamed the streaming testsuites.
Added tests for testing network input stream.
Minor fixes.