HashReduce based on java-util-hash-map

This is a first step for the hash-aggregator, corresponding issue: https://github.com/stratosphere/stratosphere/issues/82

This implementation is a ReduceOperator based on a hash-table, instead of sorting the elements.
It is just a prototype because it is still using the java-util-hash-map, and furthermore the CostEstimation is just 0 for now. 

This PR is based on the PR for dividing the reduce and group reduce operators(https://github.com/stratosphere/stratosphere/pull/726), only commit bb5640a is the HashReduce code. I just opened the PR to get feedback on the current code and how I should proceed with a stratosphere-hash-map implementation and adjusting the CostEstimation.