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 GameScreens' constructor
* Moved common Player and Enemy fields to Character
* Moved Spawner to Entities
* Added GameScreen general initialization feature

Signed-off-by: David Dorfman <wiz3kid@gmail.com>
Merge branch 'master' of https://github.com/Prismik/TDPlatformer

Conflicts:
	Game/Game/Screens/GameplayScreen.cs
	Game/Game/Screens/PauseMenuScreen.cs