[streaming] Updated streaming groupBy operators to allow grouping on field expressions
[streaming] Aggregation rework to support field expression based aggregations for Pojo data streams
[streaming] Pojo wordcount example added to streaming
[FLINK-1224] [streaming] getExecutionEnvironment method fixed to work properly for StreamExecutionEnvironment
Refactor I/O Manager and readers to interfaces, add implementation for asynchronous I/O