Support more of FlxSprite's properties in FlxSpriteGroup, closes #560

This required an interface for `FlxPoint` (`IFlxPoint`) and a helper point for the sprite group (`FlxPointHelper`).

The following properties are added to `FlxSpriteGroup`:

- `immovable`
- `angle`
- `facing`
- `offset`
- `origin`
- `scale`
- `velocity`
- `maxVelocity`
- `acceleration`
- `drag`
- `scrollFactor`

This resolves #560.