Sound Effects, Score System and more improvements!

Some sound effects have been added such as:

• Background music.
• Shoot sound.
• Power up pickup sound.
• Explosion sound when enemy destroys.

Also some graphics effects has been added like:

• Effect of smoke has been added when the cannon shoots.
• Added explosion effect when enemy is destroyed with a little bit of screenshake.

Pause menu added:

• Press ENTER to pause the game!

Added SCORE system:

• As more targets you hit with one shoot (including power ups) the more multipliyer you will have to calculate that shoot score. 
• So try to hit as many targets as you can with single shoot to multiply your score!.

Game ends when killing the last enemy, not when the time expires:

• Before after one minute, the level was completed. Now, enemies will spawn for 1 minute and when you will the last one, the level will be completed.

Fixed some minior bugs.