Average temperature and humidity for zones and dcs

Since racks are in zones and dc's it is useful to have a summary of the
temperature and humidity of the racks provided in the dc and zone stats
Removing the hardcoded units for Temperature averages
Adding units of measure to rack sensor templates
Including the units on the tooltip now
Forgot to add the auto temperature label to zones
Merge pull request #477 from m0zes/master

Average temperature and humidity for zones and dcs