New models

Oxygen, Hydrogen, CO2, Methanal
Added a bunch of collectables

also fixed methane (needed to update main scene to), made collectables
asset folder
forgot to add this oxygen prefab update
Merge remote-tracking branch 'origin/master' into user_story_15

Conflicts:
	unity-project/Assets/Scenes/main_game.unity
from master
Merge pull request #15 from themolecularmoose/user_story_15

User story 15