Moved GameplayScreen and LevelSelectScreen into *.screen package
Added exception handling for illegal state (calling addSpawn after start)
Abstracted elements from GameplayScreen into BaseScreen class

This should make it a lot easier to add new screen.
Implemented LevelSelectScreen with a placeholder Level1 button
Merge branch 'master' of https://github.com/siketh/COP4331_Project
Added state for when player loses
fixed bug