display 5m SGV delta between SGV and arrow

This displays the change in SGV since the last data point, like the Pebble app does.  I put it in between the SGV and the arrow, at 50% of the size of the SGV.  If colors are enabled, the delta will be grey if it's <= 5 mg/dL, yellow if it's >5 and <=10, and red if it's >10.

If we want to do this in dev, it should probably be an optional off-by-default feature, so someone will need to add a pref for it.

![screenshot 2014-11-11 20 51 51](https://cloud.githubusercontent.com/assets/6982790/5005514/a34e19a2-69e4-11e4-8605-b8c50df9afd1.png)