Forecast: Do not display day if temperatureMin/Max DNE.

Some days do not have temperatureMax or temperatureMin in the API. This results in a NaN° being shown in our results (also, we can't make a graph with a NaN).

This fixes #335.

![screen shot 2013-11-17 at 11 49 41 am](https://f.cloud.github.com/assets/81969/1558764/cc28e4e4-4fa8-11e3-86e1-617ec744102b.png)

![screen shot 2013-11-17 at 11 49 50 am](https://f.cloud.github.com/assets/81969/1558765/d10acbe4-4fa8-11e3-85d2-c3da566ea3e0.png)

This is how Forecast.io handles it, too:
![screen shot 2013-11-17 at 11 54 50 am](https://f.cloud.github.com/assets/81969/1558768/ff9c1602-4fa8-11e3-9cde-699f5830c8f9.png)