Pause Menu, Red Blood Cell Burst, Score Keeping

Added a basic pause menu. I decided it looked cooler when instead of
freezing time completely, it reduced it to 2% of regular speed. To make
this work I had to make Red Blood Cell generation/movement and the
Background scrolling effect dependent on time. Also had to make Virus
Handling disabled during Pause.

Cleaned up Global so that it handles states more explicitly. Using it to
handle Pause and Game Over states now.

STILL NEED TO MAKE GAMEOVER MENU SEPARATE FROM HIGH SCORE MENU

Made Red Blood Cells expand when they are being infected. Makes them
look like they pop. Made new Cell Burst Prefab for RBC bursting. Made it
yellow and more dramatic than the green Virus Spawn Bust Prefab.

Made score depend on the cumulative number of viruses you have had to
prevent having a -infinity score at Game Over.