Changes method of populating asteroids

Instead of adding a set amount of asteroids, the game adds asteroids
until the sum of their values exceeds a set limit. At the start of the
game there is no difference, but later on the same method can be
called again to 'top off' the playing field with new asteroids, and
the limit can be gradually increased.
Adds random asteroid population

randomAsteroid takes a type param which is a string corresponding to
the name of the asteroid in Asteroid.defaults object (medium, large,
mega...). If the type is 'random' it randomly selects between them.

Game constructor changed to pass {type: 'random'} to populateAsteroids