Added text property to FlxButton

I added this for these reasons:
- It makes sense for an user perspective, if he wants to change the text of the button the first thing he will try is to use ```button.text = "something else";```
- Less error prone. Before to change the text you would need to do ```button.label.text = "something else";``` but that can fail if ```label``` is null:
```haxe
var button = new FlxButton(0, 0); // label is null because there is no text
button.label.text = "change"; // say Hi to your favorite exception :)
```