Improvements for Samza engine

1) Configurable HDFS home directory for SAMOA. If it is not set, the default directory is $HOME/.samoa (particulary: /user/<user-name>/.samoa)
2) Add kryo registration for 2 AMRules-related classes (Perceptron, TargetMean)
3) Change source delay: from between instances to between batches of instances
4) Allow Samza EPI without an output stream