Made some classes private[stremaing] and deprecated a method in JavaStreamingContext.

Classes `RawTextHelper`, `RawTextSender` and `RateLimitedOutputStream` are not useful in the streaming API. There are not used by the core functionality and was there as a support classes for an obscure example. One of the classes is RawTextSender has a main function which can be executed using bin/spark-class even if it is made private[streaming]. In future, I will probably completely remove these classes. For the time being, I am just converting them to private[streaming].

Accessing underlying JavaSparkContext in JavaStreamingContext was through `JavaStreamingContext.sc` . This is deprecated and preferred method is `JavaStreamingContext.sparkContext` to keep it consistent with the `StreamingContext.sparkContext`.