Rotation near complete, next block display added.

Restructured HTML, made game centered and look prettier.
Restructured how functions in the index file worked. Changed gameLoop
function to call several other functions (one, completeRows(), not
implemented yet)

In Block.js, changed how collision is handled - no longer using offsets,
but going through each cell and checking them individually.
Also implemented rotation, but it is not a complete solution, with a
temporary canRotate function.