add physics. start with gravity

Also:

 * add jumper example, jumps whenever you chat
 * fix `game` event to fire correctly
 * add `respawn` event which fires when you die or change dimensions
 * Block instances have a `boundingBox` property, which is currently either
   `solid` or `empty`.
bot.game reserved for game settings

 * `bot.game.spawnPoint` moved to `bot.spawnPoint`.
 * `bot.game.players` moved to `bot.players`.
fix broken test
refactor tests
*WIP*