Bands events

An events to inform about band start and band end and, the latter is also fired at a player stop/pause.

Builder Syntax:

```
b().....onstart(function(ltime, duration) { console.log(ltime, duration); });
b().....onstop(function(ltime, duration) { console.log(ltime, duration); });
// aliases: onbirth/onborn and ondeath/ondie
```

native syntax:

```
var elm = new anm.Element();
elm.on(anm.C.X_START, function(ltime, duration) { console.log(ltime, duration); });
elm.on(anm.C.X_STOP, function(ltime, duration) { console.log(ltime, duration); });
```