Add sound effects

[McSinyx]

• [x] Background music/noise: perfectly loopable
• [x] Game-over: robotic dying moan
• Enemy movement: sliding sound, about 1/3 second in duration
• [x] Long-range hit: short, sci-fi/techno themed
• [x] Missed (long-range) attack
• Hero movement: short and perfectly loopable
• [x] Close-range hit: short and perfectly loopable

The last 2 effects will be looped infinitely. Volume will be used to indicate the current speed/damage in the current (frame) update.

Edit: This is not an FPS, movement sound will be unnecessary complication which will also make the audio bloated. Hit sounds should be stereo (pygame doesn't support real 3D sound, so this should be enough).

[McSinyx]

This is being solved on the new branch bachelor.