Reducing the difficulty a little

The game was a litte harder than I wanted, so I made some adjustments to deal with that.

The adjustments are as follows:

- The points required to advance level now starts at 40 instead of 80, so to get to the boss now, you just need 5.120 instead of 10.240;
- The enemy wave generator will generate less enemies, it was 10 times the level, now its 8 times the level;
- The interval between each shot of the starting weapon have been slightly decreased.

Some other minor tweaks on the code were made that have no significant impact on the gameplay.

There are some planned features, like a pause menu when you hit "esc" and a difficulty selector.