Added a level selection view controller.
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.
Fixed grid model
Fixing aesthetics and original issues with pull request
Fixing merge conflicts in grid txt file
Merge pull request #13 from zsweedyk/feature-level-selection

Added a level selection view controller.