Lives, levels, respawning and more

* The player now has lives
* The player now respawns
* The levels load automatically (only works with one level for right now)
* The core will persist throughout because it houses the GUIManager, GameManager, and EnemyManager
* Improved GUIManager so that the text now fades out instead of just popping out
* The camera smoothly lerps between where the player is
* Play the game through the Levels scene in the Level scene folder (copy it if you want to make changes/test stuff)

@ckhamash @adao1123