Final set of Spark Streaming changes

Here the major changes at a high-level

- Major changes to the streaming scheduler and checkpointing modules to handle driver-failure recovery correctly with file input stream.
- Added information on checkpointing, fault-tolerance and Java API to the streaming programming guide 
- Added Twitter input stream to StreamingContext
- Changed names of few functions (countByKeyAndWindow => CountByValueAndWindow, *network*Stream => *socket*Stream)
- Fixed missing functionalities in the Java API to make them consistent with the Scala API
- Removed unnecessary stuff from Kafka input stream as well as other fixes (contributed by Sean McNamara)
- Added actor and zeroMQ input streams along with support for arbitrary receivers (contributed by Prashant Sharma)
- Added Twitter + Algebird examples (contributed by Nick Pentreath)