Added Spring class. Fixed reason why World wasn't pre and post updating (Stage didn't have an exists property).

src/physics/Spring.js

@@ -0,0 +1,66 @@
+/**
+* @author       Richard Davey <rich@photonstorm.com>
+* @copyright    2014 Photon Storm Ltd.
+* @license      {@link https://github.com/photonstorm/phaser/blob/master/license.txt|MIT License}
+*/
+
+/**
+* Creates a spring, connecting two bodies.
+*
+* @class Phaser.Physics.Spring
+* @classdesc Physics Spring Constructor
+* @constructor
+* @param {Phaser.Game} game - A reference to the current game.
+* @param {p2.Body} bodyA - First connected body.
+* @param {p2.Body} bodyB - Second connected body.
+* @param {number} [restLength=1] - Rest length of the spring. A number > 0.
+* @param {number} [stiffness=100] - Stiffness of the spring. A number >= 0.
+* @param {number} [damping=1] - Damping of the spring. A number >= 0.
+* @param {Array} [worldA] - Where to hook the spring to body A, in world coordinates, i.e. [32, 32].
+* @param {Array} [worldB] - Where to hook the spring to body B, in world coordinates, i.e. [32, 32].
+* @param {Array} [localA] - Where to hook the spring to body A, in local body coordinates.
+* @param {Array} [localB] - Where to hook the spring to body B, in local body coordinates.
+*/
+Phaser.Physics.Spring = function (game, bodyA, bodyB, restLength, stiffness, damping, worldA, worldB, localA, localB) {
+
+    /**
+    * @property {Phaser.Game} game - Local reference to game.
+    */
+    this.game = game;
+
+    if (typeof restLength === 'undefined') { restLength = 1; }
+    if (typeof stiffness === 'undefined') { stiffness = 100; }
+    if (typeof damping === 'undefined') { damping = 1; }
+
+    var options = {
+        restLength: restLength,
+        stiffness: stiffness,
+        damping: damping
+    };
+
+    if (typeof worldA !== 'undefined')
+    {
+        options.worldAnchorA = [ game.math.px2p(worldA[0]), game.math.px2p(worldA[1]) ];
+    }
+
+    if (typeof worldB !== 'undefined')
+    {
+        options.worldAnchorB = [ game.math.px2p(worldB[0]), game.math.px2p(worldB[1]) ];
+    }
+
+    if (typeof localA !== 'undefined')
+    {
+        options.localAnchorA = [ game.math.px2p(localA[0]), game.math.px2p(localA[1]) ];
+    }
+
+    if (typeof localB !== 'undefined')
+    {
+        options.localAnchorB = [ game.math.px2p(localB[0]), game.math.px2p(localB[1]) ];
+    }
+
+    p2.Spring.call(this, bodyA, bodyB, options);
+
+}
+
+Phaser.Physics.Spring.prototype = Object.create(p2.Spring.prototype);
+Phaser.Physics.Spring.prototype.constructor = Phaser.Physics.Spring;