Rocket refactoring + starting some features / fixes

[LeMayday]

• improved code organization
  • created GCFueledVehicleEntity class
  • Rocket, Lander, and Buggy now inherit form appropriate classes
  • added / reorganized related interfaces a little
• fixes / feats
  • rocket does not hang when exited (#322)
  • projectiles bounce off all GCVehicles
  • started working on rocket inventory

[TheMCSpacePro]

Thanks for dealing with that stupid hangin' rocket bug!