Added shooting of balls and enemy death animation

* BobController.java
- add code to add balls to world when fire key is pressed

* Enemy.java
- Removed useless EnemyState.DEAD
- added boolean to keep track of when an enemy is hit
- added abstract method to handle when an enemy is hit

* Justin.java
- Added hit and dying animations
- added a check to remove dead enemies and update score
- implemented hit method for Justin
- stop enemy movement while in hit state

* World.java
- Added attributes for storing score and balls fired
- Modified populateDrawableItems() method to fix bug for disappearing
enemies

* GameScreen.java
- added a call to update method for each ball in the world

* MainMenuScreen.java
- fixed bug where loading screen wasn't loaded before game screen

* WorldRenderer.java
- added method for drawing balls
- made the code compatible with new populateDrawableItems() method
- added drawing of ball bounds in debug renderer

* Ball.java [NEW]
- new class for handling balls shot by bob
Added Bob's death, hit and proper shooting mechanisms

* GameScreen.java
- added touch region for shooting for Android devices

* MonsterAssault.java
- added GameOverScreen

* BobController.java
- added a pool for balls for optimization
- changed constructor to accept the game object as parameter for
changing of screens
- added code to handle Bob's death and switching to game-over screen
- added code to simulate when Bob is hit
- added a check to prevent fast shooting by repeatedly tapping the key
- implemented fast shooting by long-pressing the key
- repositioned ball's initial positions based on which side they are
fired from

* Ball.java
- added default constructor
- new method setBall() for setting the parameters for Ball objects (used
with pool for Ball objects)

* Bob.java
- added boolean attribute hit
- new method hit to inflict damage to bob

* Justin.java
- Invoke Bob's hit method upon collision with Bob

* WorldRenderer.java
- made frame durations public for using the values in BobController
- added frame durations and animations for hit, dying and firing
long-press
- modified drawBob() method to draw hit, firing and dying frames

* GameOverScreen [NEW]
- Blank screen, yet to be implemented