Exposed some audio related functions and added audio example

[Abev08]

Exposed LoadWAV function.
In LoadWAV and LoadWAVIO:

• changed audioBuf argument from **uint8 to *[]uint8 to be clearer and removed the necessity for user to use unsafe package to get the audio data slice,
• added copy of the data allocated by the internal function and freed the original data ensuring that the data is freed when not needed,
• made audioBuf and audioLen arguments optional:
  • if audioBuf is provided but the audioLen is not, the audio data is copied into audioBuf and the length of the audio data is length of the audioBuf slice,
  • if audioBuf is not provided but the audioLen is, the function returns length in bytes of the audio data (less useful option),
  • if both are not provided, the caller gets just the format of the audio data (not really useful).

Exposed GetAudioStreamGain and SetAudioStreamGain.
Exposed GetAudioStreamFormat and SetAudioStreamFormat.
Exposed GetAudioStreamDevice.
Added example that uses most of these functions.

[JupiterRider]

@Abev08 Thank you very much. Can you just please add the License of the wav File or at least tell me where you got it from?

[Abev08]

I have had this file for a very long time, I don't remember where I got it from :/
Sorry, I didn't think about the license of the sound sample I used.
Maybe it would be better to use another sample instead of this one?