Add powerproduction to powerusage

Original request here:
fabianblom/Toon-Android#12

When using /happ_pwrusage?action=GetCurrentUsage

{
   "result":"ok",
   "powerUsage": {"value":276, "avgValue":373.76},
   "powerProduction": {"value":0, "avgValue":0.00},
   "gasUsage": {"value":0, "avgValue":0.00}
}

for powerusage (field powerUsage), can you show also powerProduction into the PowerUsage but with minus sign.
If powerProduction > 0 then powerUsage will be 0. Same other way around.

Of course it will be better to change the text "Power usage" to "Power production" when displaying powerProduction.