Auto-targeting

Ships now auto-target an in-range enemy
Emplacements turn to face that enemy
Added zoom factor to LevelScreen, Emplacements only fire when within 10 degrees of target
AIShip - class that controls AI ships. For now, picks a movement spot near the target to move to
Added mapBounds to LevelScreen, clamped camera and background drawing to these
Fixed up the aiming issue - problem was in the GameObject.rotateToward method