Addressed issue #8

Changes:
* Renamed "Sprites" to "Entities"
* Renamed "GUI" to "HUD" (Heads-up Display)
* Registered AudioManager as a service (Issue #8)
* Removed AudioManager from MainMenuScreen's constructor
* Moved common Player and Enemy fields to Character
* Moved Spawner to Entities
* Added GameScreen general initialization feature

To do (for this commit specifically):
* Finish converting use of AudioManager from component to service.