ArcadePhysics.Body.onOverlap is a new Signal that is dispatched whenever the Body overlaps with another Body. Due to the potentially high volume of signals this could create it is disabled by default. To use this feature set this property to a Phaser.Signal: `sprite.body.onOverlap = new Phaser.Signal()` and it will be called when an overlap happens, passing two arguments: the sprites which collided.
ArcadePhysics.World.separateCircle is a new method that handles all circular body collisions internally within Arcade Physics (thanks @VitaZheltyakov)
All of the Arcade Physics internal methods, such as `collideGroupVsSelf`, `collideSpriteVsSprite` and so on, have been updated to work with circular body shapes (thanks @VitaZheltyakov)
ArcadePhysics.Body.onWorldBounds is a new Signal that is dispatched whenever the Body collides with the world bounds, something that was previously difficult to detect. Due to the potentially high volume of signals this could create it is disabled by default. To use this feature set this property to a Phaser.Signal: `sprite.body.onWorldBounds = new Phaser.Signal()` and it will be called when a collision happens, passing one argument: the sprite on which it occurred.
The Arcade Physics overlap method would return false if two bodies were overlapping but neither had any velocity (i.e. they were embedded into each other)
Two immovable bodies would never set their overlap data, even if an overlap only check was being made. As this is useful data to have this has been changed. Two immovable bodies will still never separate from each other, but they _will_ have their `overlapX` and `overlapY` properties calculated now.
* Body.setCircle allows you to define a Body as using a circle to collide with instead of a rectangle. You can set the radius of the collision circle and an offset.
* Body.render now renders both circle and rectangle body shapes to the Debug canvas.
* World.intersects has been updated to support both circle and rectangle body shapes, and supports quick-paths for circle vs. circle and rect vs. rect checks.
* World.circleBodyIntersects is a new method that checks for intersection between a Body that has been defined as a circle, and a normal rectangle based Body. This is used internally by World.intersects, but exposed for direct calls as well.
Sprites with Arcade Physics bodies that had `collideWorldBounds` enabled would be moved to the wrong position if you restarted a State (or swapped to a new State) that reset the world bounds (thanks @vulvulune #1775)
Physics.Arcade.sort now calls one of four functions: sortLeftRight, sortRightLeft, sortTopBottom and sortBottomTop. Each of which takes 2 Sprites as arguments.
Physics.Arcade.sort now doesn't bail out if the Group contains a mixture of physics and non-physics enabled objects, as the Group hash is now only ever populated with physics enabled objects. Also the sort comparison functions no longer return -1 if the bodies are invalid, but zero instead (#1721)
