Tidy up updating.
Systems are updated over an interval of time.
Systems have orbits.
Randomize orbit periods.
Slow down orbits.
Planets rotate.
Merge branch 'master' into orbits-in-systems

Conflicts:
	Space.xcodeproj/project.pbxproj
	Space/SPACEMyScene.m
Merge branch 'master' into orbits-in-systems
Merge branch 'master' into orbits-in-systems
Merge pull request #20 from MecoKing/orbits-in-systems

Orbits in systems