Added support for changing the background color based on the retrieved temperature (Celsius).

The following commits will add the ability to change the background color based on the retrieved temperature (on data load). The CSS classes for several temperature ranges are based on Celsius.

Specific Fahrenheit classes could be added to the stylesheet. The colors should probably get a more general name in that case and be called from both the Celsius and the (new) Fahrenheit CSS classes. Than a check on the format (data.format, I think) should be added in the CoffeeScript file. I could also added these features, if needed.