Fix visibility check on map view. Fixes #463

The following check was failing on the first load, so autozoom didn't
work:

3f4a30e054/src/view.map.js (L232)

src/view.map.js

@@ -53,7 +53,7 @@ my.Map = Backbone.View.extend({
 
   initialize: function(options) {
     var self = this;
-    this.visible = true;
+    this.visible = this.$el.is(':visible');
     this.mapReady = false;
     // this will be the Leaflet L.Map object (setup below)
     this.map = null;