WIP: Sound support

**Work in progress**

Sound support for Lively.

The current implementation uses the *XAudioServer* library which suposedly works across different browsers. The sound classes are ported from @Daningalls with an additional *fluent interface* for playing a melody on an instrument.

The following example code plays a tiny A minor loop with two *PluckedSound* instruments.

```javascript
require('lively.Sound').toRun(function() {

var plucked = new lively.Sound.PluckedSound();
var instrument = new lively.Sound.Instrument(plucked);

// bass line
instrument.play("a2", 4)
          .then("e3", 2)
          .then("c3", 2)
          .loop();

// lead melody
instrument.play("c5", 1)
          .then("e4", 0.5)
          .then("d4", 0.25)
          .then("e4", 0.25)
          .then("a4", 0.125)
          .then("c5", 0.125)
          .then("b4", 0.25)
          .then("d4", 0.25)
          .then("e4", 0.25)
          .loop();
});
```