New graphics

Removed drop shadows, which looked out of place, and added
check_blue.png for colourblind people.
Added feedback sound effects

Added two sound effect files, tink and funk, for correct and incorrect
answers respectively.
Sound effects and colourblind update

Sound effects are now played when you get an answer right or wrong.
Added a global parameter, $colourblind, which when set to True, causes
the program to use a blue checkmark and blue text for correct mini test
answers (instead of green).