[SPARK-7799][SPARK-12786][STREAMING] Add "streaming-akka" project

Include the following changes:

1. Add "streaming-akka" project and org.apache.spark.streaming.akka.AkkaUtils for creating an actorStream
2. Remove "StreamingContext.actorStream" and "JavaStreamingContext.actorStream"
3. Update the ActorWordCount example and add the JavaActorWordCount example
4. Make "streaming-zeromq" depend on "streaming-akka" and update the codes accordingly

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #10744 from zsxwing/streaming-akka-2.