[Issue 52]: addCombination doesn't work in DimensionTimeBucketOperator

The following pull request has following changes
1. Fix for issue https://github.com/DataTorrent/Malhar/issues/52
2. Test case for issue https://github.com/DataTorrent/Malhar/issues/52
3. Enhancement for MultiWindowDimensionAggregation Operator. Instead of recalculating sum at the end of window we are doing sum at every process.