allows gravity to be a point rather than a direction

src/core/Engine.js

@@ -294,12 +294,36 @@ var Engine = {};
             if (body.isStatic || body.isSleeping)
                 continue;
 
+            if (gravity.isPoint === true) {
+                gravity = _calculateGravity(gravity, body.position);
+            }
+
             // apply gravity
             body.force.y += body.mass * gravity.y * 0.001;
             body.force.x += body.mass * gravity.x * 0.001;
         }
     };
 
+    /**
+     * Caluclates gravity based on point and body position
+     * @method calculateGravity
+     * @private
+     * @param {vector} point
+     * @param {vector} body position
+     * @return {vector} normalized gravity vector
+     */
+    var _calculateGravity = function(point, position) {
+        var x = point.x - position.x;
+        var y = point.y - position.y;
+
+        var length = Math.sqrt( x * x + y * y );
+
+        return {
+            x: x / length,
+            y: y / length
+        };
+    };
+
     /**
      * Applys `Body.update` to all given `bodies`.
      * @method updateAll