Asteroid placement system

[SharktasticA]

As discussed in the tutorial, we need an random asteroid placement system.

For the first stage, this involves:

• Randomly selecting an asteroid type (use a couple of placeholders for now)
• Randomly generating a starting location for the asteroid

Later on, this will need to be expanded to:

• On generation, push asteroid in a direction close to Earth, but with a random variance to 'shake' things up