Created AI difficulty levels

Created two levels of difficulty for the AI:

• Difficult AI - in this mode the enemy ship will use predictive aiming to aim where it thinks you are going to be. It also makes better use of the ridiculous rapid fire mode.
• Easier AI - in this mode the enemy ship will aim at your current location and not do any prediction. This is still moderately effective as both ships are moving, and so it hits more than I had anticipated.

Other minor updates:

• Changed player ship icon
• Added menu sounds