Added atoms, collisions, and combinations

Aka, a big commit that I should've divided...
Added more molecules. Minor refactoring.
Added player input

Looks awesome so far. Really need to tag this commit so I can come back
to it easily.
Atoms now die after some time

A 5 second lifespan to be exact. This is starting to slowly turn into a
game.
Added scoring and GUI

Now it's a game. Just need to add start and game over...
Added title and game over screen.

Now, it's a game.