Merge pull request phaserjs#818 from georgiee/fix-intercative-pixi

Do not overwrite PIXI.InteractionManager so I can use PIXI standalone with Phaser on the same page

src/Phaser.js

@@ -75,7 +75,8 @@ var Phaser = Phaser || {
 
 };
 
-PIXI.InteractionManager = function () {
-    // We don't need this in Pixi, so we've removed it to save space
-    // however the Stage object expects a reference to it, so here is a dummy entry.
-};
+// We don't need this in Pixi, so we've removed it to save space
+// however the Stage object expects a reference to it, so here is a dummy entry.
+// Ensure that an existing PIXI.InteractionManager is not overriden- in case you're using your own PIXI library.
+
+PIXI.InteractionManager = PIXI.InteractionManager || {};