Forecast: added temperature unit switcher (again)

When searching the weather, I noticed that one of my past contributions, #457, was lost with `bttf`. So, I added it back with the text in the `.zci__metabar__secondary-text`. When clicking on the link, the unit will switch and the temperature values will be updated. One can hover over the link to find out what it does.

![screenshot from 2014-06-07 14 38 47](https://cloud.githubusercontent.com/assets/4411471/3209502/ff5400ec-ee72-11e3-8945-f8a9a8a1f767.png)
![screenshot from 2014-06-07 14 42 46](https://cloud.githubusercontent.com/assets/4411471/3209514/a350138e-ee73-11e3-8bfb-4948f57141f7.png)