Calculate min, max, avg cpu for logical operators (was sum)

Resolves [MLHR-1005](https://malhar.atlassian.net/browse/MLHR-1005)
![screen shot 2014-04-24 at 11 00 39 am](https://cloud.githubusercontent.com/assets/1390651/2793008/620f8974-cbda-11e3-807b-3ae51414a3d8.png)