Simplified the TMX implementation by removing useless base classes and related inheritance

and additionally, it also clarifies the documentation, and should help
with ticket #140
ticket #110 : added a `getCurrentTrack()` function
ticket #110 : added a global volume control

and also added a volume parameter to the `me.audio.playTrack` function
(to be in line with the `me.audio.play` one)
ticket #110 : stop the current playing track when `me.audio.disable()` is called
rewrote the `me.audio.playTrack` function to reuse the `me.audio.play` one
ticket #110 : added mute(all)/unmute(all) audio functions