Return value when setting crafty.asset(key, value)

To make things easier
e.g.

```javascript
	var prefixMusic = "/music/";
	var musicAssets = [		
		Crafty.asset("music1.mp3", prefixMusic + "music1.mp3"),
		Crafty.asset("music1.ogg", prefixMusic + "music1.ogg")
	];
	Crafty.audio.add("music1", musicAssets);
```

remake of #595