Don't caclulate swap if none exists

Systems without an active swap report 0/0% utilization which causes an error to be displayed when executing the `ee system status` command. 

Added a conditional to check for no-swap and the status screen now displays 'N/A' instead of 0%...

![screen shot 2014-05-04 at 3 10 32 pm](https://cloud.githubusercontent.com/assets/1479206/2873808/d46474ca-d3bf-11e3-82e1-9aa519549838.png)