fix(meter_categories): reorder result like a tree according relation parent/child

#67, #68

![screen shot 2014-11-11 at 11 40 42 am](https://cloud.githubusercontent.com/assets/2622152/4991054/dfab2b4e-6997-11e4-8dda-9c46705319c2.png)

![screen shot 2014-11-10 at 5 02 57 pm](https://cloud.githubusercontent.com/assets/2622152/4977888/390e598e-68eb-11e4-9d80-0ed8d60c4394.png)