adjusted code to fulfill PEP-8 requirements

[kajarosz]

• fixed sound files extensions in .py file to be compliant with actual files extensions (changed from .wav to .mp3)
• changed classes names to CamelCase
• simplified boolean if statements e.g. "if goblin.visible:" instead of "if goblin.visible == True:"
• simplified intervals statements e.g. "0 < bullet.x < 500" instead of "bullet.x < 500 and 0 < bullet.x"
• added missing spaces and blanklines
• removed redundant parenthesis
• changed CamelCase to snake_case in functions
• removed variable music = pygame.mixer.music.load() as load method does ot return anything. Using standalone load method in sufficient.