2D Shooter enhanced - MU course

Week 4 assignment: 2D Shooter game enhanced.

I made 3 changes and a little more:

• Added C# code to be able to activate the display of current lives, from 3 at the beginning of a level, reducing by 1 as per damage and health script.
• Added a background to the score and current lives information, re-aligned text displays and all enemies in a battle formation in so we see them all well.
• Removed the level 3 button in the main menu to keep only two levels.
• Made level 1 easier with speed 0.5 for the chaser enemy (to make it easy for beginners) and level 2 a bit faster with speed increased.  

PS: I fixed a bug that was preventing from using the pause (the page index had to be set to 0). Pause can be activated with the ESC key.

Credits: MU package