Fixed usage and other comments in different streaming examples, and add JavaRecoverableWordCount example

1. Fixed usages and comments on various streaming examples. Also that of standalone mode client. 
2. Added JavaRecoverableWordCount as an example of using JavaStreamingContext.getOrCreate.
3. Made small improvement in StreamingContext.getOrCreate to automatically attach the checkpoint directory. This is what the intended behavior should be.
4. Despite 3, updated RecoverableNetworkWordCount and JavaRecoverableWordCount to attach checkpoint directory to a newly created context (setting checkpoint directory is sort of idempotent) to make sure that the example is compatible with Spark 0.9's StreamingContext.getOrContext() which does not attached the checkpoint directory.