1.38 update

* Added support for wheel joint
* Added panning property to Scene to determine when the scene is being
touch panned
* Box2D begin and end contact callbacks now call both objects that were
affected
* Added corner_radius property to rectActor to render rounded rects
* Added support for stroke colour and thickness to rect, arc, polygon
and label actors
* Added support for scene and actor layer ordering. Set layer via _layer
property and not layer variable to ensure that re-sorting takes place
* Added new shadow properties all actor types (shadow, shadow_x,
shadow_y, shadow_colour, shadow_blur)
* Added support for composite operations to actors
* Added support for Scene onKeyPres, onKeyDown and onKeyUp events
* Added new demos keys, mouse joint and bitmap animation
* Added new Gradient class that handles gradient brushes
* Rect, Arc and Polygon actors can now render gradient brushes at any
angle, angle is specified by new grad_rotation property