Fixed bug where only the first level would play, no matter which button was pressed.
The problem was twofold. First, we simply forgot to remove a call in the LevelViewController's viewDidLoad method that started level number 0. Second, it seems calling startLevelNumber in the LevelSelectionViewController's prepareForSegue method doesn't work; this seems to be because it's called before viewDidLoad. To work around this issue, I added a public property to the LevelViewController that the LevelSelectionViewController sets and made the LevelViewController call startLevelNumber in its viewDidLoad.
Merge branch 'feature-level-selection' into feature-level-addition