Multiline text

I added support for multiline text. Use _\n_ character to separate lines.
You can also set an alignment of the text (left, center or right). It doesn't make sense for single line text.
I wasn't sure if it should be the last parameter or the 3rd one (but I thought that alignment is more important then stroke).

```javascript
var textSample = new PIXI.Text("Pixi.js can has\nmultiline text!", "50px Snippet", "white", "center", "#FFFFFF", 6);
```