* added initial openal support:
   * audio_controller: responsible for setting up and managing the openal context and audio_store
   * audio_store: contains all loaded audio files (currently only 8-bit and 16-bit wavs are supported)
   * note: audio/openal support can be disabled by defining FLOOR_NO_OPENAL (no premake option yet); openal is enabled by default on all platforms except iOS (will have to check later if I can use OpenAL-soft or have to use the system framework)
 * updated SDL2 requirement in floor_version.hpp (must be at least 2.0.2)
 * version bump to v0.1.1-d2