Separate wave header decoding

[dedobbin]

Hey, i wanted to obtain some wave file metadata, so i refactored the loading process to allow loading of only the header. This could also be used to implement a partial load in the future.

I didn't implement it for AIFF files (yet) because i don't know if its possible in the same way. I never worked with that format before. It didn't seem a big problem because the loading of different file types is pretty isolated.