Merge pull request #31 from krux02/master

Implemented some missing functions in sdl_mixer
Add darwin cgo build commands for Mac OSX targets.
Updated README

Signed-off-by: Jacky Boen <aqiank@gmail.com>
Merge pull request #32 from TomMurray/master

Add darwin cgo build commands for Mac OSX targets.
Updated README

Signed-off-by: Jacky Boen <aqiank@gmail.com>
Merge branch 'master' of https://github.com/jackyb/go-sdl2