Changes in Analytics Backend.

- In the BackendService.py:
     + Updated SparkStreamer call with new parameters.
- In SparkStreaming.py:
   + Added 'GetAggregerations' and 'ApplyThresholds' methods
   + Added 'window_size', 'win_slide_duration', 'time_stamp_col' and 'thresholds' parameters.
- Added new messages.
- Updated the 'RunSparkStreamer' call wth new parameters.