Removed an unused import
DefaultTank now have constants for its starting max/min-speed
Moved TankSpawner to the model package

TankSpawner is no longer a singleton class but is now an object stored
in World.
World now holds a TankSpawner object

It made more sense to store it in World (maybe even GameController...?)
since it's easier to handle if you reset the game.
Removed unused imports
A new class for keeping track of rounds, winner etc.

This class is work in progress. But it should function for now. It
keeps track of a lot of things:
scoreLimit - not yet implemented
rounds - how many rounds that will be played
playerLives - how many lives the players should have each spawnTime -
the spawn time for all the players
roundCounter - which round the game's on
roundTime - how long each round will be
roundTimer - how long the current round has left
gameTimer - how long the current game has left

The overall idea for now is to add two different gamemodes. One where
the winner is determined by a certain scoreLimit, from kills. The
second gametype, the winner will be the last player to stand, score
will be awarded depending on how long you live each round.
GameController now holds a GameConditions object

The newGame method, in GameController, now requires several additional
arguments.
Edited to fit the latest change to TankSpawner

TankSpawner is no longer a singleton. It's from now on an object in the
current World.
Added an active boolean to player to see when players are eliminated
Play adjusted to fit the updates to GameController and TankSpawner