Bounce the arrows when a dashboard value changes

Requested in #414; this makes the red and green arrows – and the "delta" value – bounce in when a change occurs:

![output_optimized](https://cloud.githubusercontent.com/assets/4383/2671025/a4000f08-c0d5-11e3-9cc6-6994195fd4aa.gif)

@dennisschoenmakers What do you think? Is that what you wanted?