Made default grid 7x7

Grids with odd dimensions work better because then you can miss a moving
obstacle and then hit it on your next pass. With even dimensions, you
always either hit or miss it.
Only allow controlling blocks when state == "game"
Add colorful background
Make blocks move every other beat than the first

This was always the intended behaviour, but the code caused them to not
move on the first beat of every loop of the music.
Add FIX note about music length and extra beat