Lots of change and improvement. :(

I made a lot of changes to the code and almost didn't commit anything long time ago. So here's a pull request with some commits with lots of changes and improvements.

* A new "Advanced Options" page that lets you set the "Buffer Size." (On Windows it requires quite a high value, but it's quite delayed, but on Mac, you can set it as low as 32 samples.)
* A "haste mode" that makes the song go faster if you have high HP.
* The server can press "Enter" in game to start the game.