Make logoURL be a property of IDE

This makes it easier to embed Snap! in another site where the JS code
and the html file are in different directories.

gui.js

@@ -221,6 +221,7 @@ IDE_Morph.prototype.init = function (isAutoFill) {
     this.projectName = '';
     this.projectNotes = '';
 
+    this.logoURL = 'snap_logo_sm.png';
     this.logo = null;
     this.controlBar = null;
     this.categories = null;
@@ -487,7 +488,7 @@ IDE_Morph.prototype.createLogo = function () {
     }
 
     this.logo = new Morph();
-    this.logo.texture = 'snap_logo_sm.png';
+    this.logo.texture = this.logoURL;
     this.logo.drawNew = function () {
         this.image = newCanvas(this.extent());
         var context = this.image.getContext('2d'),