Performance improved by Combine function in recommendation

The Combine function improve the performance in the recommendation.

For example, in the next picture, the Combine receives 80000 pairs, and only emits 1650. In the big scenarios, the performance improves a lot.

![beneficio_combine](https://cloud.githubusercontent.com/assets/7209542/4684922/82880d4e-5635-11e4-8f0e-7fa95cf77e95.png)

Combine function removes in the Map local node the irrelevants pairs, and the network traffic (and time) between Map and Reduce is more efficient

Regards