Added exit door to level

* LevelLoader.java
- Load block textures before loading level
- added creation of closed door to the level

* Block.java
- added 2 block types: DOOR_CLOSED, DOOR_OPEN
- block textures are now stored in Block class as static members
- added an attribute to store texture associated with the type of block
- added a method to load textures from given Texture Atlas

* WorldRenderer.java
- removed loading of block textures

* Level.java
- removed exitPosition and corresponding getters and setters

* BobController.java
- Ignore collision with closed door block
- Added TODO for inserting code for handling level completion

Other changes:
- added exit door in level-1.png
- added closed and opened door textures