Added pause feature, new font and relative positioning

* BobController.java
- release all keys when switching to next level to fix bug

* GameScreen.java
- Separated GUI rendering code into new class: GuiRenderer
- refactored renderer to worldRenderer
- added boolean to know when game is paused
- added changing of Bob's side when accelerometer values are within the
threshold value
- changed behaviour of back key to pause the game instead of going back
to main-menu screen

* [NEW] GuiRenderer.java
- moved existing GUI rendering code from GameScreen.java
- added drawing of FPS when debug mode is on
- added Bob's face next to health bar
- repositioned health bar
- repositioned score due to new font

* LoadingScreen.java
- added loading of game paused image
- changed position of loading percentage relative to screen height

* GameOverScreen.java
- changed position of score relative to screen height

* MainMenuScreen.java
- changed position of play and exit buttons relative to screen height
- adjusted touch coordinates according to new font, this time relative
to screen height so that it works well with all device resolutions

* New Assets
- replaced bitmap font with a better looking one
- added image for drawing bob's face next to health bar
- new image for displaying when game is paused