New rules!

- I've finally read the Tetris rules and changed so the Tetrominos
always spawn with the flat side down, instead of with a random rotation.
- Switched to using arrow keys for moving the blocks.
- Adjusted the game dimensions and the way they are set. They can now be
changed in tetris.js without breaking the game or layout.
- Updated color scheme.
- Adjusted the CSS to center everything correctly.