added consolidation selector for each metric

When doing large time window request ,grafana uses maxDataPoints to reduce the received data and graphite server computes a "consolidated" version of the real data, else it can be impossible to plot in real time on the client side .

The problem of this method is that graphite ( if not changed by hand ) consolidates by averaging data, and sometimes  dependant on the metric nature this could  be  erroneous ( by example when counting hits).

Graphite give us the option to change the consolidation method with function consolidateBy(), this PR enables the automatic change of consolidation method when needed.

In the next picture you can see the exactly the same metric ploted with each consolidation option (avg,sum,max,min). 

![image](https://cloud.githubusercontent.com/assets/5883405/4623864/62f6a26e-5357-11e4-91f5-a0a3f056671c.png)


YES !!! is the same metric !! queried with maxDataPoints !! this example could give us the importance of maintain visible for user the consolidation method that grafana will use.