Fixed bug where objects weren't appearing. Also added game introduction and time limit.

I was able to fix the bug where certain objects were disappearing.
Turns out I just forgot to make the ForceField objects that were member
variables for those objects ignore the objects they were centered on.

I also added a text introduction to the game that explains what
GameFrame is/does and what the objective of the tech demo game is. It
also tells the user the controls. In addition to this, I made the game
have a time limit before a game over screen shows the score you were
able to reach in 3 minutes. You can then reset the game and play again
by pressing enter. You can also now quit out of the game by pressing
escape.