Enable "Fish Generator"

[tedbarnett]

We want random fish to appear, swimming up the river and offscreen. To do this:

• Enable random sets of 1-3 fish to appear on the right side of the stage
• Once generated, each fish should swim upstream at a randomly-determined speed (within a defined range)
• The fish should occasionally change speed and direction up to 20% (dart about, slow down)
• To start, we will just generate BlackBass fish
• When more models are added, they should be able to be generated too