Refactor away Game#reset()
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
Merge pull request #57 from scmx/level-2

Level 2