Setting forecast min and max

Instead of using the initial forecast temperature, set the min and max temperatures in the forecast.
Current extension:
![old](https://f.cloud.github.com/assets/5084679/1378741/62208cd0-3ad8-11e3-8988-92359b7f0aec.png)

With patch applied:
![new](https://f.cloud.github.com/assets/5084679/1378742/64da6ac2-3ad8-11e3-8218-9a7e615f1e1a.png)