Refactored GridColumn.getAggregationValue.

The function were always iterating all rows and saving the results on a local variable to be used in only 4/6 of the time, causing a lot of overhead.

Its now wrapped in a local function and is called only when needed.

Here are the Batarang and ngStats screeshots from before and after the modification.

Before:

![monitorangularforeachpadrao - ngstats](https://cloud.githubusercontent.com/assets/5025985/5142524/c2efa216-716a-11e4-8b72-3903e9aba681.PNG)
![monitorangularforeachpadrao](https://cloud.githubusercontent.com/assets/5025985/5142525/c2f44d8e-716a-11e4-8cb7-5ea22d38ee7c.PNG)

After:

![monitorangularforeachcomfuncaolocal -ngstats](https://cloud.githubusercontent.com/assets/5025985/5142531/ddbf8124-716a-11e4-8ff7-16d1ec82e359.PNG)
![monitorangularforeachcomfuncaolocaltodosagregadores](https://cloud.githubusercontent.com/assets/5025985/5142545/009199c6-716b-11e4-80a7-78d2d359234a.PNG)