Added screens and an asset manager

* MonsterAssault.java
- added attributes for storing various screens
- set game to catch back key press so that the game is not killed

* GameScreen.java
- Added constructor to take a reference to our main game so that
switching screens is easier
- handle back key press to go back to main menu screen

* LevelLoader.java
- now uses asset manager to load texture atlas

* WorldRenderer.java
- now uses asset manager to load texture atlas

* Justin.java
- now uses asset manager to load texture atlas

* New Classes:
- SplashScreen.java
- MainMenuScreen.java
- LoadingScreen.java

* Other changes:
- added a bitmap font (villa.fnt)
- added images for splash-screen, main-menu screen and loading screen
(same as main-menu screen)