Added enemy AI and combat i.e enemies shoot back / chase player

TODO:
- add player hitbox / player takes damage from enemies
- add random weapon 'drops' for each stage and allow player to pickup
- add extra features for shots - scatter / bounce etc..
- textures & xml files for 5 enemies and 5 weapons
- player / enemy bullet textures
- floor / wall / steps textures
- move to next level when all enemies are defeated
- start screen - push space to start the game
- interface - player hp / current score / weapon + attributes
Cleanup