Added sprite

The sprite is a texture with some drawing properties.

## Anchor Point
The anchor point is a point in normalized texture coordinate. It is referenced when transforming or rotating.

## Position
Where the anchor point is in the world coordinate.

## Rotation
The sprite can rotate around anchor point.

## Scale
The sprite can also scale along X/Y direction.

## Flip
The sprite can flip vertically or horizontally.

## Children
The sprite can have children. The child's position is relative to it's parent.

Once this PR is merged, I'll send a PR of an example with sprite into `piston-example`. You can see the usage in my [swing-copters-rs](https://github.com/Coeuvre/swing-copters-rs/blob/master/src/main.rs#L63).
Should be 'its children' - English is a silly language :P