Added level codes

As the number of levels has grown, I've been asked whether I plan on including a 'save' feature - starting again from scratch can be a bit dishearterning. However, I'm not sure I want to provide the ability to save and keep on trying until you get it right.

Instead, I've introduced a level code feature. At the start of each level, a 4-letter code is displayed. These are worth making a note of - codes can be entered from the start screen to begin the game from that level. Of course, you won't have all the weapons and ammo from your previous attempt, but on the plus-side you have all your lives and you don't need to repeat the old levels.

A couple of other additions:

• You can now find an upgraded blaster - this replaces your starting gun. It shoots further, faster, and does more damage.
• Added a "controls" screen so you can see what all the keyboard controls are. Note that I haven't covered the game pad controls yet - I've still got a lot more testing to do on that front!