Pre-class demo release

Making this release as a known pretty-stable commit before the end-of-CS493 in-class demo.

Since last release,

• re-enabled collisions (they were disabled because of some PhysicsEntity refactoring related to Habitat implementation)
• added tracking of heading and spin for entities
• entities now don't look ugly when they're far away from the Sun
• trails are now drawn relative to a user-settable reference
• added fuel and throttle to the habitat
• habitat now has engines and reaction wheels, allowing it to spin and fire engines!!!!!!!
• and these can be controlled by the GUI!!!!!!!!!!

W, S control engines
Q, E rotate the habitat