Forecast: added temperature unit change button

Fixes #253

![image](https://f.cloud.github.com/assets/4411471/2364074/8106bdca-a65f-11e3-8941-0dd624139476.png)

Pretty straightforward really. I wasn't sure about the style of the javascript though; there wasn't a semicolon to be found :confused:. Let me know if there's anything that needs to be changed!