Sum of geom in FeatureGrid

As the title says it.

Picture:
![sum_geom](https://cloud.githubusercontent.com/assets/1792111/3979110/937bf13c-2850-11e4-9df5-483342b4a251.png)

What about
```
geomUnit: this.target.mapPanel.map.getUnits() + (isPolygon ? '²' : '')
```
?
(you gonna hate me :P)