Implement listener gain & get functions for OpenAL
Implement alIsBuffer
Implement basic stub for alEnable & alDisable
Implement alGetSourcefv
Add myself to AUTHORS
Use correct error handling for new OpenAL functions
Use correct index for alIsBuffer
Add get&set for source direction and cone values
Merge pull request #2056 from jamesu/openal_t2d_fixes

Openal t2d fixes