Added persistent level unlocks.
Replaced the LevelSelectionViewController's storyboard buttons with programmatic buttons, which get enabled or disabled based on an NSUserDefault integer representing the highest level unlocked. Updated the LevelViewController to update this integer when a level is completed.
Extracted numLevelButtons constant to the class level.
Merge pull request #26 from zsweedyk/feature-level-unlocking

Added persistent level unlocks.