Use ElectricityDisplay for human-readable EnergyMeter units.
Add power recording to networks and display to EnergyMeter
Refactor EnergyNetwork.split() to make sure lots of unneeded networks aren't created
Add an EnergyNetworkRegistry to keep track of Energy Networks and act as a singleton tick handler.
Merge pull request #179 from unpairedbracket/power-squashed

Add Power Reading to EnergyMeter