Playability improvements

In general this update should make the game more playable, since we fixed infinite speed gains and bullet calculations. Also the music won't start over in main menu, so no refresing the page to restart.

Here is the full changelog:

• menu resolution fix
• UI and Palmtree overlap fix
• updated attack UI message to say left mouse button
• resetting player values when returning to menu, both on death and on finish
• no longer instantiating a new music object every time you return to menu
• resetting gained speed in a level when the level is reset (player falls off)
• fixed bullet direction calculation