finish multi-bulb feature implementation

Implement multi-bulb feature. Things I did/changed:
1. add a lightDown method to bulb class so that bulb will be turned down when not connected
2. connected method in GameModel class will return true if all bulbs are connected
3. in GameModel class, a list bulbIndices is used to track the indices of connected bulbs, and GameViewController will ask for this list and turn on all these connected bulbs
4. level 4 is used to test the feature, but extra unit tests will be added in the future
5. I change the next button in gameviewcontroller back to backToLevel button so that I can jump between levels more freely

TODOs in the future:
1. add unit tests for this feature
2. sound effects
3. more levels with multiple bulbs