Level 2

Kolla på #56 först

##### Add Game#nextLevel() and #setLevel()
Use #nextLevel() to change to the next level
Use #setLevel() to go to a specific level

##### Add Level#goalsFulfilled()
Determines if game can move on to the next level
Add Level#allTreesDead() which returns true if any tree has hp > 0
Add Level#trollsKilled counter for current level