SpaceGate: difficulty levels

So I added in difficulty levels, now you can choose from EASY, NORMAL or HARD from the options screen (selectable from the main menu). I think that the difficulty levels should help to cater for casual players as well as hardcore.

This has been my approach throughout, e.g. the option of mouse controls or keyboard, to acknowledge the divide in the preference among players on these matters and cater for both.

In terms of gameplay, enemies shoot less frequently on easier modes, and the boss fights last less time. There is also a bonus mutliplier added to the players score after winning boss fights. The multiplier is much higher on harder modes so skilfull players are rewarded with higher a multiplier on harder modes.

All that needs to be done is to tweak a few of the attack patterns and the game will be ready to release in august.