Add bindings for SDL_Sound.

Heya!

 I'm not sure if there's an appropriate mailing list to talk about this which is why I'm getting in touch via a pull request ;)

 I recently implemented go bindings for SDL_Sound, as I want support for more sound formats in the application I'm working on. I have seen reports though about linker incompatibilities and bugs caused by trying to use SDL_Sound and SDL_Mixer at the same time, and I'm not entirely sure if that has consequences for the go bindings.

 It works fine for me, but I've been using SDL_Sound by itself.

 There's a test program included, which should probably be rewritten to not load the entire decoded samples of test.ogg into memory at once ;)

-sqweek