Major modifications to fix driver fault-tolerance with file input stream

List of changes at a high-level
- Modified FileInputDStream, Scheduler and Checkpoint* classes to achieve full driver fault-tolerance with file input stream.
- Changed countByKeyAndWindow to countByValueAndWindow and added countByValue
- Added Prashant Sharma's pluggable network receiver and akka actor input stream
- Changed networkStream to socketStream, and added new networkStream function take arbitrary network receivers 
- Added Twitter input stream to StreamingContext
- Moved Java streaming examples to examples/src/main/java/spark/streaming/examples/
- Updated streaming programming guide with information about checkpoint and faul-tolerance semantics, and fixed mistakes in the DStream operations
- Fixed bug in CheckpointRDD to not crash when the original RDD had 0 partitions
- Fixed bugs in JavaAPISuite
- Fixed Java and Scala doc comments