* added audio_source: this handles a single openal source object and currently supports background audio (no positional data) and 3D audio (positioned in 3D space with misc openal properties: velocity, reference-distance, rolloff-factor, max-distance); TODO: filter and effect support
 * added audio config settings (music/sound volume and optional device name)
 * set the distance model to AL_EXPONENT_DISTANCE_CLAMPED for now
 * clear the store when calling audio_store::destroy()