CHANGE collect system memory instead total

From erlang(3erl):

   total = processes + system
   processes = processes_used + ProcessesNotUsed
   system = atom + binary + code + ets + OtherSystem
   atom = atom_used + AtomNotUsed

   RealTotal = processes + RealSystem
   RealSystem = system + MissedSystem

Thus it makes more sense to collect system memory and sum `total` during
the display, if needed.
Merge pull request #3 from Motiejus/system_memory

CHANGE collect system memory instead total