Publish percentages of used resources.

Configured via new env var, SHH_PERCENTAGES, a comma separated list:

   - df: If poller df is on, publish total used as percentage for each
     filesystem that is reported
   - mem: if poller mem is on, publish total memory in use as percentage
   - swap: if poller mem is on, publish total swap in use as percentage.
Merge pull request #2 from apg/publish-percentages

Publish percentages of used resources.