v0.75 devlog

v0.75 devlog:

Major changes

- Replaced health icons with progressively sick avatar that has a sneeze/cough animation that plays when pathogens get through.

- Added credits page to main menu.

- Adjusted resolution of main and  normal level backgrounds to work on mobile.

- Updated engine from 4.2.2 to 4.3

- Adjusted size of touchscreen buttons to make them easier to press - larger in most cases

- Game is fully playable on Android - getting a new error on iPhone.