Forecast: switch the wind units as well

Fixes #1250, cc @jagtalon 

![screenshot - 11122014 - 07 17 57 pm](https://cloud.githubusercontent.com/assets/4411471/5021223/a111f390-6aa0-11e4-9c60-1f0f788ee1a0.png)

There is one roadblock I ran into: If the API returns fahrenheit results (`unit == 'us'`), then we can't be sure which unit speed unit to switch to. We have a choice between `km/h` and `m/s`, and I assumed the latter. Maybe this could be a setting?